The novel is about a lonely young man, Frederick Clegg, who works as a clerk in a city hall, and collects butterflies in his spare time. The first part of the novel tells the story from his point of view.
Clegg is obsessed with Miranda Grey, a middle-class art student at the Slade School of Fine Art. He admires her from a distance, but is unable to make any contact with her because of lacking social skills. One day, he wins a large prize in the football pools. He stops working and buys an isolated house in the countryside. He feels lonely, however, and wants to be with Miranda. Unable to make any normal contact, Clegg decides to add her to his "collection" of pretty, petrified objects, in the hope that if he keeps her captive long enough, she will grow to love him.
After careful preparations, he kidnaps Miranda by drugging her with chloroform and locks her up in the cellar of his house. He is convinced that Miranda will start to love him after some time. However, when she wakes up, she confronts him with his actions. Clegg is embarrassed, and promises to let her go after a month. He promises to show her "every respect", pledging not to sexually molest her and to shower her with gifts and the comforts of home, on one condition: she can't leave the cellar.
The second part of the novel is narrated by Miranda in the form of fragments from a diary that she keeps during her captivity. Miranda reminisces over her previous life throughout this section of the novel, and many of her diary entries are written either to her sister, or to a man named G.P., whom she respected and admired as an artist. Miranda reveals that G.P. ultimately fell in love with her, and subsequently severed all contact with her.
At first, Miranda thinks that Clegg has sexual motives for abducting her, but as his true character begins to be revealed, she realises that this is not true. She begins to pity her captor, comparing him to Caliban in Shakespeare's play The Tempest because of his hopeless obsession with her. Clegg tells Miranda that his first name is Ferdinand (eventual winner of Miranda's affections in The Tempest).
Miranda tries to escape several times, but Clegg stops her. She also tries to seduce him to convince him to let her go. The only result is that he becomes confused and angry. When Clegg keeps refusing to let her go, she starts to fantasize about killing him. After a failed attempt to do so, Miranda passes through a phase of self-loathing. She decides that to kill Clegg would lower her to his level. She refrains from any further attempts to do so. Before she can try to escape again, she becomes seriously ill and dies.
The third part of the novel is narrated by Clegg. At first, he wants to commit suicide after he finds Miranda dead, but after he reads in her diary that she never loved him, he decides that he is not responsible for what happened to her and is better off without her. The book ends with his announcement that he plans to kidnap another girl.