Twenty-five-year-old Philip Ashley narrates the novel, prompted to reflect on the events of the past year by his guilt over the death of his cousin Rachel Ashley. An orphan, Philip grew up on a large estate in Cornwall, raised by his beloved older cousin Ambrose. After Philip graduated from university, Ambrose began spending winters abroad as treatment for his rheumatism. On one such winter, when Philip is twenty-four, Ambrose travels to Florence, Italy. Philip soon receives a letter from Ambrose saying that he has met and married an Ashley family relative named Rachel, the former Contessa Sangalletti. Ambrose seems completely taken with Rachel, but within a few months the tone of his letters shifts: he has begun suffering from extreme headaches, and he refers to Rachel as “my torment.” Alarmed, Philip consults with his godfather and guardian, Nick Kendall, who recommends Philip travel to Italy immediately.
In Florence, Philip learns that Ambrose has recently died, allegedly of a brain tumor, and that Rachel is nowhere to be found. Swearing vengeance on Rachel, whom he believes to have somehow contributed to Ambrose’s death, Philip returns to Cornwall. Within a few weeks, Philip learns that Rachel has just arrived in Plymouth, England. Determined to question her about Ambrose’s final days, he invites his cousin Rachel to stay with him. The servants at the Ashley estate, particularly the steward, Seecombe, are kind to Rachel; unlike them, Rachel has not been provided for in Ambrose’s will, which bequeaths the estate to Philip on his twenty-fifth birthday.
Over the ensuing months, Philip and Rachel bond over their shared love of Ambrose, and Philip begins to enjoy Rachel’s presence at the house, even though he set out to hate her. He even applies to Nick Kendall, his legal guardian until his birthday, with a request to provide Rachel a monthly allowance, since no other financial provision is made for her in Ambrose’s will. When Rachel learns of this allowance, she is initially offended because she feels Philip is patronizing her. However, she eventually reconciles to the idea, and even kisses Philip (his first kiss), which stuns him.
The weeks pass, and Philip gradually becomes infatuated with Rachel. One day, while sorting through Ambrose’s old books, Philip finds an unfinished letter in which Ambrose accuses Rachel of extravagant spending and kleptomania (the uncontrollable impulse to steal). Philip is disturbed by the letter, but he ultimately puts it out of his mind. By December, Philip is wholeheartedly in love with Rachel. He decides to throw a Christmas party at the estate, and withdraws a family heirloom, a pearl collar, from the bank to give to Rachel as a present. At the dinner, Nick Kendall confronts Philip. He is concerned by Philip’s decision to give Rachel the necklace. Not only does Philip not have the legal right to remove the family jewels from the bank, since he is not yet twenty-five, but the necklace also holds traditional and superstitious value: Ashley women only wear it on the occasion of their wedding. Furthermore, Kendall tells Philip that he has learned Rachel has overdrawn her monthly allowance, and he is concerned that she may be sending money out of the country. Kendall and Philip argue, but when Rachel appears, she calmly cedes the necklace to Kendall.
As the New Year begins, Philip decides to make improvements to the house, both to compliment the additions Rachel has already made to the garden, and to annoy Nick Kendall with the extravagant bills. One day in early spring, he is summoned to the home of a tenant, to whom Philip had given one of Ambrose’s old coats as a Christmas gift. The tenant has found a letter inside the coat. In it, Ambrose writes that, ever since a miscarriage she suffered, Rachel has become increasingly reckless about money. Her friend and advisor, Rainaldi (whom Ambrose suspects might also be her lover), has also begun asking some disturbing, prying questions about Ambrose’s will. Ambrose has written a new version of the will that bequeaths the estate to Rachel, but he has left it unsigned because he is worried about her spending habits. Ambrose concludes the letter with his fears that Rachel and Rainaldi are conspiring to poison him. Torn between his loyalty to Ambrose and his newfound love of Rachel, Philip buries the letter in the woods. He does, however, question Rachel about the alternate will and learns she has a copy of it in her possession. Philip resolves to turn the estate over to Rachel on his twenty-fifth birthday (under the condition that she not remarry); this way, she will be compelled to stay with him rather than return to Florence.
Philip’s birthday, April 1, draws near. The document passing the estate to Rachel has been drawn up, and Philip has also removed the entire family jewel collection from the bank. On the eve of his birthday, Philip visits Rachel’s room and presents her with the lavish jewels. He comes close to confessing his love for her, but can’t quite find the words. In the early morning hours of Philip’s birthday, he and Rachel have sex. At his birthday dinner later, a drunken Philip announces to the Kendalls that he and Rachel are engaged to be married. Appalled, Rachel makes an excuse and diverts the conversation. Later that night, Philip and Rachel quarrel: he insists she promised to marry him by having sex with him, and Rachel maintains she gave no such promise. She claims that Philip misinterpreted her actions, as she only meant to thank him for the jewels. Philip becomes so enraged that he strangles Rachel, and demands she agree to marry him. Rachel escapes without saying anything, and the next day invites Mary Pascoe, the vicar’s daughter, to stay with her in the house for protection.
The next day, Philip discusses matters with his childhood friend, Louise Kendall, who is his godfather’s daughter. She suggests that Rachel has been after Philip’s money all along, and that he spared her the trouble of having to embezzle by transferring the entire property to her. Philip can’t bring himself to believe this. After spending some time in the rain that afternoon, Philip becomes gravely ill. Rachel spends five weeks nursing him back to health, but plans to leave for Italy as soon as he is well. Desperate to keep Rachel in England, Philip rifles through Rachel’s desk, searching for a letter he knows her to have received from Rainaldi, which he thinks might reveal something about Rachel’s plans. Instead, Philip finds an envelope filled with poisonous laburnum seeds, and realizes Rachel might be poisoning him with the tea (tisane) she makes him each night.
The following morning, Philip receives warning from one of his construction workers that the new bridge they have built over the sunken garden will not yet bear weight. When Rachel says she will be taking a walk in the garden, Philip does not warn her about the bridge. Instead, he convinces Louise Kendall to help him search Rachel’s room for proof that she has been poisoning him; they are unable to find anything. Suddenly, Philip asks Louise to ring for help, as “there may have been an accident, to Rachel.” Louise looks at him in horror, and Philip runs outside. He finds Rachel lying on the stones, under the collapsed bridge. He holds her and she opens her eyes in what Philip believes is recognition. Rachel calls Philip “Ambrose,” and then dies.