In late 19th-century Victorian England, a scientist named Sir Hugo Cunningham discovers a way to photograph the soul, which he believes manifests at the moment of death as an ethereal entity called the Asphyx. Driven by grief after the tragic deaths of his fiancée and son, Sir Hugo becomes obsessed with furthering his research. His experiments reveal that capturing an Asphyx can grant immortality to whoever controls it. With the help of his adopted son, Giles, and his daughter, Christina, he builds a complex apparatus to trap these spirits.

As Sir Hugo's obsession grows, he begins subjecting himself to dangerous experiments, believing that achieving immortality justifies any risks. The ethical boundary between scientific pursuit and moral responsibility becomes increasingly blurred. Tensions rise within the family when they realize the potential consequences of tampering with life and death. The pursuit of eternal life threatens to unravel not just their relationships but also their very humanity.