The film's plot depicts Alan, Tin-Tin, Lady Penelope and Parker representing International Rescue on Skyship One's round-the-world maiden flight, unaware that criminal mastermind the Hood is once again plotting to acquire the secrets of the Thunderbird machines. Paid agents of the Hood murder the original crew of Skyship One prior to take-off and assume their identities, entertaining the guests while scheming to lure the Tracy brothers into a trap. Meanwhile, Brains' efforts to produce a satisfactory design concept for Jeff's proposed Thunderbird 6 collide with fate when Skyship One is damaged and Alan's old Tiger Moth biplane appears to be the only hope of saving the International Rescue group and their impostor hosts.
Actors John Carson and Geoffrey Keen provide guest speaking roles, with additions to the regular voice cast in the form of Keith Alexander and Gary Files. The design of the puppets that appear in Thunderbird 6 marks a transition between the caricatures that Century 21 had used up to Thunderbirds Are Go and the realism introduced in Captain Scarlet and the Mysterons. Filming ran from May to December 1967, and the art and special effects departments collaborated to realise Skyship One as both a miniature model and a collection of themed interior designs. A number of sequences of the Tiger Moth in flight were filmed on location with a full-sized stunt plane, but a legal dispute with the Ministry of Transport regarding alleged dangerous flying by pilot Joan Hughes forced the production team to film the remaining shots in-studio with scale replicas.
In 2068, the New World Aircraft Corporation (NWAC) provides Brains, the inventor of the Thunderbird machines of the humanitarian International Rescue organisation, with an open brief to design a revolutionary aircraft. Although Brains is ridiculed when he proposes an airship for the 21st century, NWAC accepts his blueprints and builds Skyship One, which will circumnavigate the world on its maiden flight with pre-programmed stopover destinations. Alan, Tin-Tin, Lady Penelope and Parker will represent International Rescue as special guests. Brains, meanwhile, is forced to remain on Tracy Island when Jeff decides that International Rescue requires a Thunderbird 6. Contracted to design this latest addition to the Thunderbirds fleet with no specification, Brains produces a range of concepts, all of which are rejected by Jeff.
Alan and Tin-Tin travel to England in an old de Havilland Tiger Moth biplane and join Penelope and Parker. As Skyship One embarks on its round-the-world voyage, the International Rescue guests are unaware that Captain Foster and the stewards have been murdered and replaced by agents in the pay of the Hood, who is operating under the alias of "Black Phantom" and is based at the disused El Hadim airfield near Casablanca in Morocco. Codenamed "White Ghost", the impostors are not required to demonstrate technical knowledge of Skyship One since it incorporates automated systems, meaning that the trip passes without incident as the airship visits such locations as New York, the Grand Canyon, Rio de Janeiro and India.
Parker voices his suspicions about the crew of "Skyship One" to Penelope after he notes their lack of knowledge about the airship and Penelope relays this message to Alan but during the conversation uncovers a listening device in her room after Skyship One passes through a violent storm. Foster and his associates have secretly been recording and editing Penelope's speech to assemble a false transmission, which requests that Jeff dispatch Thunderbirds 1 and 2, with Brains on board, to El Hadim airfield where the Hood and his henchmen will be waiting to hi-jack the machines. Unable to get Penelope to say the detailed part of the message (landing coordinates of the airstrip) naturally, they use a fake newspaper to get her to read the final details aloud to complete the last part of the message just before their stop at the Egyptian Pyramids. When Skyship One makes its final stop in the Swiss Alps, while Foster, Alan, Tin-Tin, and Lady Penelope dine at a steam train themed inn, Parker locates the editing equipment and informs the guests, whilst they attempt to figure out what is going on the edited message is transmitted and sent to John on Thunderbird 5. Alan determines the threat against International Rescue just in time for Penelope to contact Jeff in person and warn that Thunderbirds 1 and 2 are about to be ambushed. Dispatched by Jeff in accordance with the transmission, Scott and Virgil now open fire on and annihilate the Hood's hideout.
On Skyship One, Alan, Penelope and Parker battle Foster and his associates in a gunfight and kill two of the five White Ghost operatives, but are forced to surrender when Tin-Tin is captured as a hostage. The "Gravity Compensation Machine" has also been damaged in the fighting, causing the airship to lose altitude. Crossing the English coast, it crashes into an interception mast at a missile base in Dover. With the airship balanced precariously on top of the mast and its anti-gravity field failing, it is up to Scott, Virgil and Brains to rescue all on board before it collapses onto the base. The Thunderbirds cannot get close enough without their jetwash tipping the airship off balance, and all of their rescue vehicles are too heavy. Gordon proposes that Alan's Tiger Moth, in storage at the NWAC Headquarters, is light enough to land on the airship without precipitating its fall.
Brains pilots the plane onto the top deck but is confronted by Foster and his two surviving associates. Holding Penelope hostage in the cockpit, Foster intends to abandon the others, but Alan shoots and kills Foster. The Tiger Moth lifts off again with all hanging on to either the wings or undercarriage, just before Skyship One crashes to the ground and obliterates the evacuated missile base in a chain reaction. A shootout on board the Tiger Moth disposes of the last of the White Ghost agents, but a bullet has penetrated the fuel tank and the controls will not respond to Penelope. After narrow misses with a bridge on the unfinished M104 motorway and an exhaust tower, Alan and Penelope finally manage to ditch the plane into a field with Tin-Tin and Brains unhurt, however they then realise that no-one has seen Parker since Alan shot Foster, leading them to believe he never made it off Skyship One, but a distant voice reveals that he was caught up in a tree the plane crashed through. Back on Tracy Island, Brains unveils Thunderbird 6 as none other than the repaired, repainted and revamped Tiger Moth, which all agree has proven its value as a rescue aircraft.
Despite the unexpected failure of Thunderbirds Are Go on its release in December 1966, the United Artists distributors authorised a sequel, to be budgeted at £300,000. Major production credits were unchanged from the first film: while Gerry and Sylvia Anderson scripted the film in three months and returned as producers, David Lane filled the position of director.
The plot of the ill-fated Skyship One was intended to be more light-hearted than that of Zero-X in Thunderbirds Are Go, although at the earliest production stage the focus was to be a "Russo-American space project". From an idea of Desmond Saunders, a long-standing collaborator who had an interest in aviation, the Andersons-based the plot on the destruction of the British R101 in 1930. Gerry Anderson researched airship history by reading books on the R101, the R100 and the Graf Zeppelin. The plot also emulates the Thunderbirds Series Two episode "Alias Mr. Hackenbacker", which stars another of Brains' pioneering aircraft, Skythrust.
Introducing a vintage de Havilland Tiger Moth as the new Thunderbird 6, in their script the Andersons allude to 1960s publicity for Esso, which advertised under the promotional banner of "Put a Tiger in Your Tank". A line from Virgil Tracy during the final rescue of the Skyship One occupants adapts this slogan to refer to the "Tiger" stored inside Thunderbird 2's Pod. However, no character dialogue explicitly refers to the aircraft by the full name "Tiger Moth".
Voice acting was recorded in six days at the Anvil Films Recording Studio at Denham in Buckinghamshire, where dialogue for Captain Scarlet and the Mysterons would also be recorded. Dialogue for characters returning from Thunderbirds Are Go, such as the Tracy family, Tracy Island's other residents, Lady Penelope, Parker and the Hood is, with two exceptions, provided by the same actors. Voice actors introduced in Thunderbird 6 are:Keith Alexander as John Tracy and the Narrator. John's original voice, Ray Barrett, had returned to his native Australia on the completion of Thunderbirds Are Go. Replacing Barrett in this role for Thunderbird 6, Alexander was also contracted to provide an opening narration, which describes the secrecy of the International Rescue organisation and Tracy Island. Emigrating from Australia in 1965, Alexander continued his association with the Andersons after Thunderbird 6, voicing the characters of Sam Loover in Joe 90 (1968) and Agent Blake in the final Supermarionation series, The Secret Service (1969). He also held the regular part of Lieutenant Keith Ford in UFO in the 1970s.
Gary Files as "Black Phantom". While production of Thunderbird 6 progressed, Files, another Australian actor, also provided voices for Captain Scarlet, including the recurring character of Captain Magenta. More roles followed, from supporting voices in Joe 90 to the regular part of Matthew Harding in The Secret Service. He recalls that his lines in Thunderbird 6 were intended as a test prior to Captain Scarlet and has fond memories of his casting, stating in an interview, "I was just knocked out to be working with people like Geoffrey Keen, and those other marvellous people involved."
John Carson as the replacement Captain Foster ("White Ghost"). A British actor of television and cinema, Carson's small-screen credits of the 1960s and 70s include The Saint, Man in a Suitcase, Department S and The Professionals, while his film roles include Plague of the Zombies (1966) and Taste the Blood of Dracula (1970). His appearances in The Troubleshooters attracted the attention of the Andersons. Carson's delivery of Foster's lines has led to a mistaken assertion that fellow British actor James Mason was cast for the role.
Geoffrey Keen as James Glenn, the New World Aircraft Corporation President. Also known to the Andersons for his role in The Troubleshooters (as the leading character of Brian Stead), stage, television and cinema actor Keen appeared in more than 100 films in his lifetime, including Doctor Zhivago (1965), Born Free (1966) and, with Carson, Taste the Blood of Dracula. Spanning 1977 to 1987, he portrayed Sir Frederick Gray, the fictional Minister for Defence in the James Bond series, from the films The Spy Who Loved Me to The Living Daylights.
Gerry Anderson biographers Simon Archer and Marcus Hearn suggest that Thunderbird 6 develops the character of Lady Penelope from her previous appearances in the Thunderbirds franchise. This is argued to be due, in part, to a more mature reading of the character's lines from Sylvia Anderson, who provided Penelope's voice from the first episode of the Thunderbirds television series. Archer and Hearn also praise the work of David Graham, particularly his reading of Parker's dialogue, but note that other regular characters, such as Gordon and John, appear infrequently in the film.
With the cancellation of Thunderbirds after the six episodes of Series Two, the next small-screen project for AP Films, re-branded as "Century 21" in December 1966, would be Captain Scarlet and the Mysterons. This series, which was screened from September 1967, starred a brand-new generation of Supermarionation puppets sculpted with more realistic proportions than the caricatured marionettes of Thunderbirds. However, in the pre-production stages for Thunderbirds Are Go, it was decided that audiences who had seen Thunderbirds were too accustomed to the older style of puppet for the returning characters to be upgraded to the new design. To maintain continuity, a compromise was made to produce puppets for the second Thunderbirds film which would mix traits from the two Supermarionation generations: although the heads and hands would remain disproportionately large, the marked caricature from the television episodes would be reduced.
For guest roles, puppets were mainly recycled from their previous appearances in Thunderbirds Are Go, although the Captain Foster puppet was a new addition. Puppeteer Wanda Webb recalls that Thunderbird 6 maintained a high standard in the appearance of its cast, commenting on a shot which depicts Lady Penelope asleep on board Skyship One, "I had placed the sleeping eyelids in Plasticine and made the eye shadow a little too blue. We ended up re-shooting the whole sequence." One-use puppets appear in what Supermarionation historian Stephen La Rivière describes as "a contender for the most horrific scene ever produced by Century 21": in the opening scene, the characters of the NWAC executives present gaping mouths and teeth, complete with dental fillings, when Brains' plan to design an airship sends them into howls of laughter. The decision to begin the film with a cold open and delay the title sequence and opening credits was one of Century 21's efforts to distinguish Thunderbird 6 from the preceding film.
Principal photography for Thunderbird 6 commenced on 1 May 1967, by which time Captain Scarlet and the Mysterons had already entered production. One of the Century 21 production units was transferred from Captain Scarlet to the second Thunderbirds film and shooting was completed in four months alongside that of the new television series. Special effects director Derek Meddings constructed the Skyship One model and supervised the creation of scale replicas of destinations explored in the film, such as the Great Sphinx of Giza, the Egyptian Pyramids and the Grand Canyon. The Swiss Alps shots which frame a scene set inside the fictional Whistle Stop Inn called for the FAB1 model to skate across ice, while scale figures of Alan and Tin-Tin follow the Rolls-Royce on skis. The length of movement required necessitated the construction of a special effects set that was between 40 and 50 feet (12 and 15 m) wide (the largest used for the film), filled with salt to simulate the snow-capped Swiss mountains. The film was also shot in parallel with Joe 90, as evidenced by special effects footage of the destruction of the Dover Missile Complex re-appearing in the pilot episode of that series, and a brief appearance of Sam Loover's car during the evacuation of the Complex.
The art department, headed by Bob Bell and responsible for the interiors of Skyship One, designed each room in a unique style: while the airship's Ball Room includes spherical-shaped décor, a Games Room is furnished to a dice and chessboards theme. Although Keith Wilson was attached to the production of Captain Scarlet, he contributed the design of Lady Penelope's bedroom, which is shaded a bright pink to match the colour of FAB1. In the course of filming, the floor of the Skyship One Bottle Room set ignited under the heat of the studio lights and had to be completely rebuilt. The interior of the Whistle Stop Inn is a favourite of Bell. Shooting on this set had to be carefully timed due to the presence of model trains, of a smaller scale than the puppets, which run on tracks and transport meals to the characters as their "cargo". In their biography of Gerry Anderson, Simon Archer and Marcus Hearn hold the Skyship One Gravity Compensation Room, in which the Gravity Compensators are represented by rotating metal frames, in high regard. However, Penelope's bedroom is described as resembling a "Barbara Cartland nightmare".
POV (point of view) shots were done with the Tiger Moth biplane. During one such time at the end of summer, by which time the grass around Booker Airfield had turned brown, members of the effects team rectified the colour by applying green paint. Some time later, the production team received a message that a furious farmer's prize ram (an uncastrated sheep) had died, and when the animal was cut open, there was a large amount of green paint inside it. Concluding that no one else but them were responsible for the death, the production team did a lot of explaining and funded the cost of the incident. Production manager Norman Foster gave the farmer's wife a big bunch of flowers.
The vintage de Havilland Tiger Moth appears as a scale model for sequences such as Alan and Tin-Tin's departure from Tracy Island and Brains' arrival on the top deck of Skyship One. The production team also decided to arrange a live-action location shoot in Buckinghamshire to star a full-sized biplane. Joan Hughes, an experienced pilot who had flown Supermarine Spitfires and Avro Lancasters from factories to airbases during the Second World War, was selected to fly the aircraft. Sequences that comprise live-action filming include Brains launching the Tiger Moth from a field to rescue the occupants of Skyship One, Penelope's subsequent struggle to control the aircraft, the gun battle with Foster's remaining henchmen, the near collisions with the motorway bridge and the exhaust tower, and the final crash-landing. For the later sequences, stunt dummies fixed to the Tiger Moth's wings and undercarriage represent the characters of Alan, Tin-Tin, Parker, Brains and the White Ghost agents, while Hughes doubles for Penelope in the cockpit. The location filming was based at Wycombe Air Park.
The M40 motorway, which was nearing completion at the time of production, doubled for the fictitious M104. In the run-up to the filming of the plane's near-miss with the bridge between Junctions 4 and 5, at Lane End on the High Wycombe Bypass, the Ministry of Transport and the local police ordered that in accordance with Ministry of Civil Aviation regulations, the manoeuvre be performed with the wheels in contact with the tarmac at all times. During a take, a crosswind sprang up and the drag caused by the dummies left Hughes concerned that she would lose control of the Tiger Moth if it were to hit the ground. She therefore kept the plane in mid-air while passing under the bridge, clearing it by nine feet (2.7 m) as originally planned. On another attempt, the continuing adverse conditions necessitated a second low-level glide, angering the Ministry of Transport official supervising the filming; subsequently, Hughes and production manager were arrested. Hughes, a pilot with more than three decades' flight experience by 1968, later commented that this dangerous stunt marked only the first time in her career that she had feared for her safety.
Both Hughes and Foster were prosecuted; the former was charged with seven counts of "dangerous flying", and the latter with three of aiding and abetting. However, it was not until 18 March 1968 – after production had ended – that the case against them was heard at Aylesbury Crown Court. The jury viewed the final cut of Thunderbird 6 and found both defendants not guilty on 20 March. Reported in the Daily Express below the headline "Under The Bridge Goes Lady Penelope", the acquittal inspired Foster to remark that the character had "opened the way for much greater realism in filmmaking."
In the intervening months, the Ministry of Transport had withdrawn permission for further stunts to be filmed on the M40, forcing the production team to resort to alternative methods to complete the Tiger Moth sequences. The special effects department constructed a 1⁄6-scale model re-creation, located on the Century 21 outdoor backlot to reduce lighting discrepancies, with radio-controlled miniatures of the Tiger Moth made to replace Hughes' aircraft. Ranging from six feet (1.8 m) in width to a smaller 1⁄3-scale for filming with the puppet characters, the replicas were unreliable and frequently crashed, but Anderson asserts that the production team successfully merged the full-sized and miniature shots so that it is difficult to distinguish which aircraft is piloted by Hughes and which is remote-controlled. The scale reconstruction of the M40 bridge was aligned with a backdrop of real trees and fields to simulate the intended setting as faithfully as possible. Bad weather forced the outdoor filming to run for six weeks.
Built at Hatfield Aerodrome in Hertfordshire in 1940, the DH82A Tiger Moth that appears in the film (registration G-ANFM, serial number 83604) had served in the RAF before being sold to the Association of British Aero Clubs in 1953. Since the production of Thunderbird 6, the plane has made appearances in other films, such as Michael Apted's Agatha (1978). Damaged in a crash in 1992, the repaired Tiger Moth currently forms a part of the Diamond Nine aerobatics squadron, based at White Waltham Airfield, Berkshire.
Barry Gray considered the musical score for Thunderbird 6 superior to that of Thunderbirds Are Go since its depiction of round-the-world travel provided scope for a large number of musical themes. Music was recorded in six sessions at the Olympic Studios at Barnes, London, between 1 and 5 February 1968, with an orchestra of 56 members. The main title music, described as "jaunty" in Gerry Anderson's biography, accompanies opening credits superimposed on shots of Skyship One as it sits on the NWAC airfield. The soundtrack was released in a limited edition in 2005. A rendition of the 19th-century song "The Daring Young Man on the Flying Trapeze" accompanies the aerial shots which chart Alan and Tin-Tin's departure from Tracy Island at the start of the film. Director David Lane wanted the movements of the Tiger Moth to simulate a dance in mid-air, and to this end played the song on loudspeakers from the shooting helicopter to inspire the stunt pilot.
Completed in December 1967 and awarded a "U" rating from the British Board of Film Classification on 22 January 1968, Thunderbird 6 did not premiere until 29 July, at the London Odeon Cinema in Leicester Square. To promote the film, a Lady Penelope impersonator, Penny Snow, toured Britain in a pink Rolls-Royce imitation of FAB1. Commenting on the six-month postponement between classification and release, Thunderbirds historian Chris Bentley conjectures that United Artists had lost faith in distributing the Thunderbirds franchise in light of the disappointing box office returns for Thunderbirds Are Go, and therefore intentionally shelved the sequel between January and July.
Underperforming at the box office on its general release, Thunderbird 6 went on to become a commercial failure and spoilt chances for the production of a potential third Thunderbirds film. In his review published in the Daily Mail, critic Barry Norman described the sequel to Thunderbirds Are Go as child-orientated but still a showcase of "technical excellence". In what he termed a "class-conscious" side to the film, Norman also discussed the characterisation of Parker, a manservant, as a butt of jokes, such as being the one unfortunate character to find himself stuck upside-down in a tree as the Tiger Moth crash-lands.
La Rivière theorises that the commercial failure of the film is attributable to the facts that ITC financier Lew Grade had cancelled Thunderbirds as a television series in 1966, and that, by July 1968, 18 months had passed since the screening of the final episode ("Give or Take a Million"), resulting in a loss of public interest in the franchise. He praises Thunderbird 6 for its visual work, and holds the model Tiger Moth effects in particularly high regard, writing that it represents "some of the best effects work Century 21 would ever create. It is a testament to their skill and ingenuity that, in the motorway sequence, the model shots are indistinguishable from the original." However, he questions whether the sparseness of the action sequences is a disappointment to child viewers, suggesting that the film "feels like an extended puppet version of holiday magazine programme Wish You Were Here...?" Adding to the "unfamiliar air" of the film are the characters, who La Rivière argues sound more mature than before in a development, which he partly attributes to the new casting of Gary Files and Keith Alexander in place of Ray Barrett. He also asserts that the film intentionally avoids iconic elements from the original Thunderbirds episodes, such as the Thunderbird launch sequences and the "Thunderbirds March" theme music.
La Rivière argues that the use of the Tiger Moth as the star vehicle, and the pun on the Esso promotional banner of "Put a Tiger in Your Tank", are more suited to an adult audience. Meanwhile, children "had spent the entire 90 minutes eagerly waiting for the most fantastic piece of hardware to arrive. They got an old plane." Thunderbirds historian John Marriott voices similar criticism of the Tiger Moth, writing that "the big screen was an unsuitable place for the gentle irony of steam-age technology scoring triumphantly over an array of fantasy machines." Science-fiction author John Peel is dismissive of the film in general and compares it pessimistically to the "well-made fun" of Thunderbirds Are Go. He summarises it as a "big mistake" and "a feeble last fling for a brilliant series", with a long and illogical plot, poor jokes and little action during the final rescue sequences. Responding to claims that the tone of Thunderbird 6 differs significantly from that of its predecessor, Gerry Anderson maintains that the time that had elapsed since the airing of the final television episode meant that "we were much more aware with the second one that it wasn't just a question of making a longer episode, but it was, indeed, to make something special for the cinema."
A BBC Online review suggests that the sequel to Thunderbirds Are Go is "a weak and perhaps too padded adventure" which is an underwhelming experience as a feature-length film. Despite summarising it as having more "the extended feel of a special TV episode", and describing the plot as substandard in comparison to the original episodes, Thunderbird 6 is awarded a rating of three out of five stars. In contrast, Jim Schembri of Australian newspaper The Age asserts that the plotting is strong (and superior to that of Thunderbirds Are Go), and that the film also boasts "a snappier pace, with an action climax leaps ahead of anything in the latest Bond epic." Writing in the same newspaper, Philippa Hawker notes an increase in humour, stating of Thunderbird 6 that it is "more self-consciously light-hearted but it's also more suspenseful." The Film4 website offers a three-star rating to match BBC Online, praising the production team's decision to introduce more naturally proportioned puppets and comparing Thunderbird 6 favourably to Jonathan Frakes's 2004 adaptation, which is considered a "live-action bomb". Described as "great fun" and "entertaining if antiquated", the film is also referred to as "a slice of kid-friendly cinema made for a far more innocent age."
In Regions 2 and 4, Thunderbird 6 was initially released on DVD by MGM in 2001, with special features including an audio commentary from director David Lane and producer Sylvia Anderson, the theatrical trailer, stills and production galleries. A 2004 "International Rescue Edition", including Region 1 and packaged both separately and as part of a box set with the preceding film, Thunderbirds Are Go, contains a Dolby Digital 5.1 surround sound mix and extends the 2001 special features material with three documentaries detailing the production of the film.
On May 13, 2014, Twilight Time (through their sublicensing deal with MGM) released Thunderbird 6 with Thunderbirds Are Go in a double feature Blu-ray set, limited to 3,000 copies and only available on the Screen Archives Entertainment website. The set was re-released on June 20, 2017, by Kino Lorber.