The book is about a lonely young man, Frederick Clegg, who works as a clerk in a city hall, and collects butterflies in his free time. The first part of the novel tells the story from his point of view. Frederick is attracted to Miranda Grey, an art student who he thinks is very beautiful. He admires her from a distance, but is unable to make any contact with her because of his weak social skills. One day, he wins a large prize in the pools. This makes it possible for him to stop working and buy an isolated house in the countryside. He feels lonely however, and wants to be with Miranda. Unable to make any normal contact, Frederick decides to add her to his ‘collection’. After careful preparations, he kidnaps Miranda using chloroform and locks her up in the cellar of his house. He is convinced that the girl will start to love him after some time. However, when she wakes up, Miranda confronts him with his actions. Frederick is embarrassed, and promises to let her go after a month.
The second part of the novel is narrated by Miranda in the form of fragments from a diary that she keeps during her captivity. She is scared by Frederick, and does not understand him in the beginning. At first she thinks that he has sexual motives for abducting her, but this turns out not to be true. She starts to have some pity for her captor, comparing him to Caliban in Shakespeare’s play The Tempest because of his hopeless love for her and his deformed way of thinking. She tries to escape several times, but Frederick is able to stop her every time. She also tries to seduce him in order to convince him to let her go. The only result is that he becomes confused and leaves the room. When Frederick keeps refusing to let her go, she starts to fantasize about killing him. Before she can try to escape again, she becomes seriously ill and dies, probably of pneumonia.
The third part of the novel is again narrated by Frederick. At first he wants to commit suicide after he learns of Miranda’s death. But after he reads in her diary that she never loved him, he decides that he is not responsible for her death. Finally, he starts to plan the kidnapping of another girl.