Sir Hugo Cunningham's (Robert Stephens) early 1900s experiments with photography lead to his discovery of the legendary asphyx, spirits that deliver death to every living thing. Theorizing that capturing an asphyx will render its target immortal as long as the asphyx remains immobilized, he devises an asphyx trap. He succeeds with a guinea pig, then with himself. Hugo talks his daughter Christina (Jane Lapotaire) and her lover Giles (Robert Powell) into defying death but Christina's procedure goes awry, killing her.
The grieved Giles arranges to have Hugo unintentionally kill him before Giles' asphyx can be caught, thus putting himself out of his misery and avenging his lost love. Hugo, unable to free his own asphyx (Giles cleverly sees to that), is left forever alone to suffer his guilt.
The film ends as it opened, with Hugo in modern London, dressed in rags and horribly disfigured (probably from numerous accidents or failed suicides). He is accompanied only by his equally immortal guinea pig.