In Victorian England, philanthropic scientist Sir Hugo Cunningham is a part of a parapsychological society that studies psychic phenomena. As part of their latest investigation, the men have begun photographing individuals at the moment of death; done properly, the resultant photo depicts a strange blur hovering around the body. Though the society concludes that they have captured evidence of the soul escaping the body, Cunningham is skeptical.
At a party to celebrate his recent engagement, Cunningham is making home movies with a primitive video camera of his own invention when his fiancee and son are killed in a boating accident. When Cunningham views the film, he sees that not only has he captured the blur, but that it is moving towards his son, and not away from him. From this, Cunningham concludes that the blur is not the soul but a force known in Greek mythology as an "Asphyx," a kind of personal Grim Reaper that comes for every individual at the moment of his or her death.
While filming a public execution as a protest against capital punishment, Cunningham activates a spotlight that he has crafted using phosphorus stones beneath a drip irrigation valve. Later, when viewing the film with his ward, Giles, Cunningham sees that the condemned man's asphyx was briefly held suspended in the spotlight's beam. Concluding that an individual's asphyx is an organic force and therefore subject to the laws of physics, Cunningham theorizes that some property of the energy released by the combination of phosphorus and water renders the asphyx immobile. If correct, this would mean that an asphyx could be trapped, and that an individual would be immortal so long as their asphyx remained imprisoned.
Giles and Cunningham successfully capture the asphyx of a dying guinea pig and seal it in the family tomb, beneath a spring fueled by the lake. Seeing immortality in his grasp, Cunningham tasks Giles with helping him to capture his own asphyx, deciding that his contributions to science are too important for him to pass away. Cunningham commissions the construction of an impenetrable vault door on his family tomb, with a complex combination lock as the only means of opening it; once he has captured his asphyx, Giles is under instruction to seal the asphyx inside, so that no one can ever set it free.
Using an electric chair to slowly kill himself, Cunningham summons his own asphyx; however, Giles is only experienced in capturing an asphyx with two men, and is forced to rely on his fiance, Christina, for assistance. Christina is horrified with the experiments, but agrees to participate when Cunningham tells her that he will give his blessing for the two to marry if they allow him to make them immortal.
Theorizing that imminent death, and not actual death, will summon an asphyx, Cunningham places Christina on a guillotine operated by Giles. During the experiment, the guinea pig chews through a hose pumping water onto the phosphorus stones being used to capture the asphyx. In the resultant panic, Christina is decapitated.
Despondent, Cunningham insists that Giles open the vault and free his asphyx. Giles agrees, on the condition that Cunningham first grant him immortality. Unbeknownst to Cunningham, Giles rigs the procedure, removing the phosphorus stones from the spotlight. As Cunningham attempts to gas Giles to death to summon his asphyx, he turns off the gas and turns on the oxygen to save Giles. Giles strikes a match. The resulting explosion kills Giles and destroys all of the equipment required to capture asphyxes.
Though Giles ostensibly left behind the combination to the vault on a slip of paper, Cunningham destroys it, resolving that his own immortality is God's punishment for the deaths of Giles and Christina. In an epilogue set in the 1970s, an ancient, disfigured Cunningham roams the streets of London with the guinea pig. He wanders into the path of an imminent car collision, which kills both of the drivers; a police officer responding to the scene is shocked to find that Cunningham, crushed beneath the two vehicles, is still alive.
What does Cunningham theorize will summon an ashyx?
Imminent death