In 1906, Saxton (Lee), a renowned British anthropologist, is returning to Europe by the Trans-Siberian Express from China to Moscow. With him is a crate containing the frozen remains of a primitive humanoid creature that he discovered in a cave in Manchuria. He hopes it is a missing link in human evolution. Doctor Wells (Peter Cushing), Saxton's friendly rival and Royal Geological Society colleague, is also on board but travelling separately. Before the train departs Shanghai, a thief is found dead on the platform. His eyes are completely white, without irises or pupils, and a bystander initially mistakes him for a blind man. A monk named Father Pujardov (Alberto de Mendoza), the spiritual advisor to the Polish Count Marion Petrovski (George Rigaud) and Countess Irina Petrovski (Silvia Tortosa), who are also waiting to board the train, proclaims the contents of the crate to be evil. Saxton furiously dismisses this as superstition. Saxton's eagerness to keep his scientific find secret arouses the suspicion of Wells, who bribes a porter to investigate the crate. The porter is killed by the ape-like creature (Juan Olaguivel) within. It then escapes the crate by picking the lock.
The creature finds more victims as it roams the moving train. Each victim is found with the same opaque, white eyes. An autopsy suggests that the brains of the victims are being drained of memories and knowledge. When the creature is gunned down by police Inspector Mirov (Julio Peña), the threat seems to have been vanquished. Saxton and Wells discover that images are retained in a liquid found inside the eyeball of the corpse, which reveal a prehistoric Earth and a view of the planet seen from space. They deduce that the real threat is somehow a formless extraterrestrial that inhabited the body of the creature and now resides within the inspector. Pujardov, sensing the greater presence inside the inspector and believing it to be that of Satan, renounces his faith to pledge allegiance to the mysterious entity.
News of the murders is wired to the Russian authorities. An intimidating Cossack officer, Captain Kazan (Telly Savalas), boards the train with a handful of his men. Kazan believes the train is transporting rebels; he is only convinced of the alien's existence when Saxton switches off the lights and Mirov's eyes glow, revealing him to be the creature's host. The creature has absorbed the memories of Wells' assistant, an engineer, and others. It seeks the Polish count's metallurgical knowledge to build a vessel to escape Earth. Kazan shoots and kills Mirov, and the alien transfers itself to the deranged Pujardov.
The passengers flee to the freight car while Pujardov murders Kazan, his men, and the count, draining all of their minds. Saxton rescues the countess and holds Father Pujardov at gunpoint. Saxton, having discovered that bright light prevents the entity from draining minds or transferring to another body, forces Pujardov into a brightly lit area. Possessed by the creature, Pujardov explains that it is a collective form of energy from another galaxy. Trapped on Earth in the distant past after being left behind in an accident, it survived for millions of years in the bodies of protozoa, fish, and other animals. It cannot live outside a living being longer than a few moments. The creature begs to be spared, tempting Saxton with its advanced knowledge of technology and cures for diseases. While Saxton is distracted by the offer, the creature resurrects the count's corpse, which attacks Saxton.
Saxton and the countess flee from the creature, but it now resurrects all of its victims as zombies. Battling their way through the train, Saxton and the countess eventually reach the caboose where the other survivors have taken refuge. Once there, Saxton and Wells work desperately to uncouple themselves from the rest of the train. The Russian government sends a telegram to a dispatch station ahead, instructing them to destroy the train by sending it down a dead-end spur. Speculating that war has broken out, the station staff switch the points.
The creature takes control of the train as it enters the spur. Saxton and Wells manage to separate the last car from the rest of the train. The creature tries to find the brakes to try and stop the train, but fails to even get it to slow down. The train rams through the barrier and plunges down the cliff and is destroyed as soon as it hits the bottom. The caboose rolls precariously to the end of the track before stopping, inches away from the edge of the cliff. The survivors quickly leave the caboose, while Saxton, Wells, and the countess gaze over the ravine and witness an inferno consuming the train and its unearthly inhabitants.
Horror Express was filmed in Madrid between 1971 and 1972, produced on a low budget of $300,000 with the luxury of having three familiar genre actors in the lead roles. The film was co-produced by American screenwriter/producer Bernard Gordon, who had collaborated with Martin on the 1972 film Pancho Villa (which featured Savalas in the title role).
Rumors that the train sets were acquired from the production of Doctor Zhivago (or Nicholas and Alexandra) were refuted by Gordon, who said in a 2000 interview that the model had been constructed for Pancho Villa. Filmmakers used the mock-up from Pancho Villa as the interior for all train cars during production since no further room was available on stage. All scenes within each train car were shot consecutively, the set then modified and shot for the next car. The train's departure scene was filmed in Madrid's Delicias railway station. The locomotive which pulls the train in that scene is a RENFE 141F, but later in the film, the locomotive seems to be an unidentified RENFE locomotive whose wheel arrangement is unclear.
Securing Lee and Cushing was a coup for Gordon, since it lent an atmosphere reminiscent of Hammer Films, many of which starred both of the actors. When Cushing arrived in Madrid to begin work on the picture, however, he was still distraught over the recent death of his wife, and announced to Gordon that he could not do the film. With Gordon desperate over the idea of losing one of his important stars, Lee stepped in and put Cushing at ease simply by talking to his old friend about some of their previous work together. Cushing changed his mind and stayed on.
Like all the Italian and Spanish films of the period, Horror Express was filmed mostly without sound, with effects and voices dubbed into the film later. Lee, Cushing and Savalas all provided their own voices for the English market.
The film generally received mixed reviews. On the review aggregator website Rotten Tomatoes, the film has a 57% approval rating with an average rating of 6.6 out of 10, based on seven reviews.
This film was first titled Pánico en el Transiberiano and first released as an officially selected film of the 1972 Sitges Film Festival. Director Eugenio Martín won the Critic's Award Best Script for this film. According to Martín, his native country of Spain was where the film fared worst, both critically and in terms of box office revenue. The film was received more positively in other markets where the audience was more familiar with low-budget horror films, such as Great Britain, the United States and Australia.
Montgomery Advertiser film critic Jery Tillotson gave the film a positive review, writing, "Good performances, brisk direction, and fast action moves this thriller a notch above the average shocker".
A special edition Blu-Ray/DVD release of the film was issued in 2011 by Severin Films.