In the film, Richard Harris leads a team of Naval bomb disposal experts sent to disarm several large barrel bombs that have been placed aboard an ocean liner crossing the North Atlantic. Meanwhile, ashore, the police race against time to track down the mysterious bomber who calls himself "Juggernaut."
The ocean liner SS Britannic is in the middle of a voyage in the North Atlantic with 1200 passengers on board when the shipping line's owner Nicholas Porter (Ian Holm) in London receives a telephone call from an unidentified person with an Irish accent styling himself as "Juggernaut", who claims to have placed seven drums of amatol (high explosive) aboard the ship which are timed to explode and sink it at dawn on the following day. He warns that the drums are booby-trapped in various ways and that any attempt to move them will result in detonation and offers that technical instructions in how to render the bombs safe will be given in exchange on a ransom being paid to him of £500,000. As an indication of his seriousness he then sets off a demonstration attack with a series of small bombs on the ship's bridge, which injure one crewman. Unable to order an evacuation of the ship's passengers via lifeboats due to rough seas besetting it, the shipping line's management is inclined to yield to the ransom demand, however when the police are called in British government officials inform the company that if it does so they will withdraw the company's operating subsidy in line with the Government's policy of non-appeasement of terrorism.
Instead, a Royal Navy officer, Lt. Cmdr. Anthony Fallon (Richard Harris), leading a bomb-disposal unit is dispatched, arriving on the scene by air transit and parachuting into the sea, with orders to board the ship and defuse the barrel-bombs before the deadline. Meanwhile back in London, Supt. McCleod (Anthony Hopkins), whose wife and two children happen to be holidaying on board the ship, leads Scotland Yard's investigation against the clock to capture the criminal master-bomber.
After an attempt to defuse one bomb by remote-controlled robot fails, setting it off and damaging the ship, Fallon decides to split up his team with each man working simultaneously on each of the remaining devices at different points around the ship, Fallon going first with each stage of the defusing operation and informing his men of each move by radio link, with the aim that if he fails and his bomb explodes, his men will know what went wrong and continue the process onwards, with his second in command taking up the lead, until the devices are disarmed. However, if two more bombs go off, the ship will sink. Fallon proceeds to disarm the bomb he is working on, apparently successfully, with his men following each step. However, it contains a hidden secondary mechanism and one of his men, close friend Charlie Braddock (David Hemmings), accidentally triggers it, resulting in his death when it explodes, causing further damage to the ship. A disillusioned Fallon at this point abandons the operation and advises the ship's captain, Alex Brunel (Omar Sharif), to advise the shipping line to pay the ransom to avoid any more carnage, but when negotiations with Juggernaut break down at the last minute, Fallon is ordered by the captain to continue disarming the remaining bombs.
Meanwhile, an extensive police search back in London captures the bomber posing as Juggernaut, who is revealed as an embittered former British military bomb-disposal officer, Sidney Buckland (Freddie Jones). He is escorted to the police situation room. After interrogation, he agrees to tell Fallon - whom he knows personally, having trained him as a junior officer - by radio-link how to disarm the bombs as time is running out and the dawn detonation is fast approaching. However, in doing so, after some hesitation when Fallon appeals for his clemency on the basis of their former comradeship, Buckland betrays him and his own chance for redemption and gives fatally false direction. Fallon, sensing he is being misled, does the opposite of what he is told, disabling the device. The rest of the bomb-disposal unit swiftly follow his example, and the ship and its passengers are saved.
Richard Alan Simmons' script was inspired by a real life bomb threat against the Queen Elizabeth 2 in 1972, which resulted in Special forces (one SAS, two from the Special Boat Squadron and a Welsh bomb disposal expert of the Royal Army Ordnance Corps) being parachuted into the Atlantic to board and search the liner, as dramatized in the film. The film was the second produced by David V. Picker after he left United Artists, following Lenny. Originally Bryan Forbes was to direct, with Simmons producing and Richard Harris to star, with the film going to start in January 1974.
Bryan Forbes left the project, however, as did his replacement, Don Medford. Picker then turned to Richard Lester, with whom he had made a number of films at United Artists. On taking over the film, Lester completely rewrote the script with writer Alan Plater.
The film's writer/producer, Richard Alan Simmons, was so unhappy with the reworked script that he had himself credited as Richard DeKoker on the finished film.
The film was shot mainly aboard the real cruise ship TS Hamburg. The German vessel had recently been sold to the Soviet Black Sea Shipping Company and renamed TS Maxim Gorkiy. Before the Soviets began operating the ship for paying passengers, the British production company chartered the ship.
Advertisements were run in British papers, soliciting extras who would take a lengthy cruise in the North Sea for free, but with the knowledge that the ship would actually seek out the worst possible weather, as the story demanded seas too rough for the lifeboats to be lowered, trapping the passengers on board.
The ship's charter was negotiated at a set rate in February 1974, while oil prices were continuously skyrocketing due to the still-ongoing 1973 oil crisis. As a result, the Soviets, who paid the vessel's operating costs during filming, ended up losing money on the deal.
Some interior filming was completed on stages at Pinewood Studios. Location shooting was also done in and around London. A room at St Thomas' Hospital in Lambeth overlooking the River Thames doubled as the office of the Managing Director of the shipping line.
The 2005 UK DVD release used the alternate title Terror on the Britannic. The film was released on Blu Ray by Kino Lorber on September 9th, 2014.