In 1906, Professor Alexander Saxton (Christopher Lee), a renowned United Kingdom anthropology, 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 Transitional fossil#Missing_links in human evolution. Doctor Wells (Peter Cushing), Saxton's friendly rival and Royal 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 and 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 a Poles Count and Countess 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 within, which then escapes the crate by picking the lock.
The creature finds more victims as it roams the moving train, each victim being 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, 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 life that inhabited the body of the creature and now resides within the Inspector. Father 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.
