When Philip Ashley's much-loved (and rich) cousin Ambrose dies, he is convinced that Ambrose was murdered by his new wife Rachel to inherit his wealth. But when he meets Rachel and falls in love with her, he knows that his suspicions must have been unfounded. But were they, or is Rachel just trying to use Philip to get at the estate Ambrose left to him instead of to her? And will she murder him next?

In the mid-1800s, orphan Philip Ashley is reared by his cousin Ambrose in a mansion on the Cornish coast of England. Ambrose is devoted to Philip, but when the youth is twenty-three, Ambrose decides to visit Italy to revive his flagging health. Philip and neighbor Louise Kendall bid farewell to Ambrose, who does not return in the spring, as he had planned. Instead, Ambrose writes to Philip that he has met a distant cousin, Countess Rachel Sangalletti, a half-English, half-Italian widow. Philip is distressed when he receives additional letters describing Ambrose's marriage to Rachel and his subsequent illness. Finally, Ambrose writes that he does not trust Rachel, who he suspects is trying to kill him. Although Louise's father Nicholas informs Philip that a brain tumor killed Ambrose's father, Philip dismisses his supposition that a tumor is making Ambrose delusional. Philip travels to Italy, but by the time he arrives, Ambrose has died and Rachel has left. A servant's description of Ambrose's death and Rachel's friendship with an Italian lawyer, Guido Rainaldi, heightens Philip's suspicions, and he confronts Rainaldi. The lawyer assures Philip that Ambrose died from a brain tumor, and that the doctors had feared that the tumor would make him paranoid. Philip is still uneasy, even after Rainaldi reveals that Ambrose left his estate to him, to be kept in trust until his twenty-fifth birthday. Philip then visits Ambrose's grave and vows that whatever pain Rachel inflicted on him will be repaid in double. Later, after Philip returns home, Rachel comes to England with Ambrose's possessions. Kendall, who is the executor of Ambrose's will, asks Philip to look kindly upon Rachel, as she is now penniless. Philip offers to let Rachel stay at the Ashley mansion, although he rudely does not greet her until long after her arrival. The headstrong Philip is astonished by how young and lovely Rachel is, and by how much she knows about the Ashley family. After a few days, Philip has been won over by Rachel's graciousness and confesses his former hatred of her. Rachel forgives him and soon after, Philip tells Kendall that he is giving Rachel £5,000 a year. Kendall protests Philip's generosity, but Philip insists that as Ambrose's widow, she is entitled to it. Meanwhile, Rachel stays on at the mansion, and encourages Philip's infatuation with her. At Christmas, Philip gives Rachel a valuable family necklace, but Kendall insists that she return it. Philip, angered at being treated like a child, is even more upset when Kendall alleges that Rachel is overdrawing her bank account to send money to Italy. Philip, who has become obsessed with Rachel, refuses to hear anything bad about her, and tries to be pleasant to Rainaldi when he visits. Later, Rainaldi reveals to Philip that Ambrose had written a new will leaving everything to Rachel, but that he died before signing it. On the day before his twenty-fifth birthday, Philip asks Kendall to write a document transferring the Ashley estate to Rachel. Despite Kendall's misgivings, at midnight, Philip presents the document to Rachel along with the family jewels. Rachel thanks Philip passionately and implies that she will marry him, but the next night, when Philip announces their engagement, she denies it. Rachel rejects Philip's next proposal, and coldly states that her earlier passion was due to his gifts. Enraged, Philip begins to strangle Rachel but stops before doing her serious harm. Soon after, Philip talks with Louise, who reveals that Rachel interrogated Kendall about the remarriage clause in Philip's document, stating that she will forfeit the Ashley estate if she remarries. When Philip returns home, he learns that Rachel, afraid of his temper, has invited a minister's daughter, Mary Pascoe, to stay with her, and that they are planning a sunken garden near a chasm on the estate. Later, Philip becomes gravely ill with meningitis, and Rachel nurses him, giving him tisanes and other herbs. During his delirium, Philip imagines that he and Rachel are married, and when his fever breaks, he calls himself her husband. Philip's recovery progresses over several more weeks, until finally, he learns from a gardener that Rachel is returning to Italy. Rachel, who was meeting with Rainaldi in the nearby town of Plymouth, returns and when Philip confronts her, admits that she will leave soon. Rachel then reveals that they are not married, and the disappointed Philip notices that she has a recent letter from Rainaldi, postmarked in Plymouth, even though she had told him that the lawyer was in Italy. Determined to read the letter, Philip breaks into Rachel's room, but the packet he retrieves contains only poisonous seeds. Horrified, Philip becomes convinced that Rachel killed Ambrose and tried to poison him. On the day of Rachel's departure, Philip confides his suspicions to Louise, and while Rachel goes to inspect the sunken garden, they sneak into her room to find Rainaldi's letter. The letter indicates that Rachel might be innocent, however, and that she genuinely cares for Philip. Philip then realizes that in order to reach the garden, Rachel must cross a bridge that the builder had deemed unsafe. Philip runs to the site but arrives too late, for Rachel has fallen to the rocky ground below. After Philip rushes to her side, Rachel sadly asks him why he did it, then dies. Later, as he stares at the sea, Philip ponders his torment over Rachel's guilt or innocence.