Victorian scientist and philanthropist Sir Hugo Cunningham (Robert Stephens) believes his experiments in photographing the dying at the very moment of their passing have somehow captured images of the victims' souls leaving their bodies. But when he unfortunately records on moving film the accidental death of his son and his fiancée, he discovers the apparition he has captured on film is moving toward the victims rather than away from them.

Sir Hugo theorizes that it must be the mythological spirit known to the Ancient Greeks as the Asphyx, thought to appear just before the moment of death to claim the living. His theory is proven to be correct when he inadvertently, but only temporarily, prevents one such creature from reaching the body of man being executed at a public hanging. Realising that if he can trap and hold an Asphyx indefinitely, he can render its owner immortal, Sir Hugo becomes obsessed by his quest for the secret to everlasting life - a quest that will threaten the very lives of those closest to him.