Helen Vaughan was born when an English scientist, Dr. Raymond performed an experiment on the brain of a young woman named Mary, in order to open her inner eye and make her come into contact with a parallel reality invisible to ordinary mortals. The experiment is successful, but the girl is raped by the god Pan, lord of that world, who makes her pregnant and leads her to madness. The woman dies after giving birth to Helen.
Many years later Helen marries a rich English gentleman named Herbert, who will seduce and "corrupt in body and spirit", ending up reducing him to poverty. Following a meeting between Herbert (now reduced to a homeless man) and his old friend Villiers, the latter, assisted by Clarke (protagonist of the story) and another man named Austin, will carry out some investigations discovering not only the marriage between Helen and Herbert, but also the complicity of the two in causing the death of another wealthy gentleman. Herbert is later found dead (presumably killed by Helen).
Intrigued, Villiers begins to investigate Helen's past and discovers that as a child he spent much of his time in the woods near his home, taking other children on long walks in the twilight and disturbing the parents of the city. One day, a little boy who had stumbled upon her "playing on the grass with a strange naked man" had become hysterical and later, after seeing a Roman statue of a satyr, permanently insane. Furthermore, Helen has an unusually strong friendship with a neighboring girl, Rachel, who leads several times in the woods. On one occasion, Rachel returns home shocked, half-naked and inconclusive: shortly after explaining to her mother what happened to her (which is never revealed in history), Rachel returns to the woods and disappears forever.
Helen disappears, meanwhile, for some time, presumably participating in disturbing orgies somewhere in the Americas. Eventually he returns to London under the pseudonym of Mrs. Beaumont, followed by a series of suicides. Villiers and Clarke, each of whom learns the true identity of Mrs. Beaumont, unite and confront Helen in her home, persuade her to hang herself, and Helen has a very abnormal death, transforming herself between the human and the beast before she finally dies.
- Although her father is considered the main antagonist of the novel, Helen is the one who causes the most damage to the protagonists and the men she seduces.