The Collector tells the story of Frederick Clegg, a man in his mid-20s who grows obsessed with Miranda Grey, a beautiful teenager whom he watches from afar. Clegg is an amateur entomologist and is especially devoted to collecting butterflies. After he wins a lot of money in a sports betting system known as the football pools, he takes the steps necessary to kidnap Miranda, though at the time he tells himself that he is not necessarily preparing to abduct her. By this time Miranda is a 20 year-old art student in London. Clegg buys a secluded house in the country and readies a small basement room as an inescapable cell; he also buys a van with which to follow and snatch Miranda. Eventually, after determining her habits and discovering where she lives, he kidnaps her one night as she is walking home in London.
Part 1 of the novel is told from Clegg's perspective, first as he prepares to abduct Miranda and then once he has her in his power. Clegg drives Miranda to his house after chloroforming her to subdue her. He forces her into the basement. For the next month, the two engage in a battle of wills, with Miranda frequently attempting to escape and Clegg explaining that all he wants is for her to love him. Miranda comes from a higher social class and has had a better education than Clegg; she frequently tries to educate him, though at other times she subjects him to her contempt. Clegg promises to free Miranda after a month of captivity, but goes back on his promise when Miranda tells him she does not love him. She tries to escape after Clegg extends her term of captivity, and for the first time since Miranda's abduction Clegg must chloroform her to keep her in his grasp.
Eventually Miranda tries to last thing the thinks might free her: seducing Clegg. After a disastrous sexual encounter, Clegg loses all respect for her, revealing the extent of his neurotic mentality in the process. Yet he begins to force Miranda to pose for nude photographs for him; these offer the only sexual gratification he can experience. Soon, though, Miranda develops a cold which becomes a severe chest infection, probably pneumonia. Clegg refuses to get a doctor, fearing discovery, and Miranda's condition worsens. Clegg keeps repeating that what eventually happened to Miranda is not is fault.
The perspective for the narration switches in Part 2, which is told from Miranda's viewpoint and formatted as a journal she keeps during her captivity. Much of her writing mirrors the events that occur in Part 1, but she also reminisces a good deal about her life back in London, especially her affection for an older artist, G.P. Miranda thinks about how much G.P. has influenced her and concludes that her time in Clegg's basement has changed her for the better.
Miranda is alternately kind to Clegg and exasperated with him, now conciliatory and now furious. She desperately wants to be set free. While at times she feels defeated, by the end of her narration she is full of life and ready to experience the world outside. Her section ends as she descends further into sickness, beginning to rave and worrying that she will die.
Part 3 switches back to Clegg's narration. He reveals that he never got a doctor for Miranda and that, after an agonizing several days of severe pneumonia, Miranda died. Clegg contemplates killing himself, thus lending their story have a tragically beautiful ending, much like the conclusion of Romeo and Juliet. In Part 4, however, Clegg has a change of heart. He buries Miranda without much emotion and sets his sights on a new victim, one who will be more pliable once captured.