Ambrose Ashley is the owner of a large country estate on the Cornish coast and guardian to his orphaned cousin, seven-year-old Philip. Ambrose and Philip run the estate together and live a harmonious, bachelor lifestyle in an all-male household. On Sundays, Philip's godfather, Nick Kendall, and his daughter Louise come to lunch as do the Reverend Pascoe and his family. Life is good apart from a few health problems that determine that Ambrose must spend the winter in warmer climes. As the damp weather approaches, he sets off for his third winter abroad and this time chooses Italy.
Now in his twenties, Philip misses Ambrose but receives letters from him, telling of his journey and then saying that he has reached Florence and met up with a cousin of theirs called Rachel. Ambrose relates that, when Rachel was young, she had married an Italian nobleman called Cosimo Sangalletti, who had been killed in a duel, leaving her childless and in a precarious financial position. Ambrose's letters reveal that he enjoys Rachel's company and that he spends much time with her.
In the Spring, when Ambrose would normally be planning a date for his return home, he sends a letter to Philip, announcing that he and Rachel are married and have no immediate plans to return to Cornwall. Philip is numb with shock and ashamed that he cannot be pleased for Ambrose, while everyone else seems delighted for the happy couple and full of questions about what Rachel is like. Louise chatters on about the changes that Rachel will make to the house; and, when Philip snaps at her, she asks him if he is jealous. Then Philip's godfather asks him if he has any plans for the future, bearing in mind that, if Ambrose has a son, Philip will no longer be his heir. Another letter arrives from Italy saying that there is a 'tangle of business' relating to Rachel that Ambrose is spending a lot of time and money sorting out and that they need to stay in Florence. Philip is relieved that he can continue running the estate for the time being.
Gradually the tone of Ambrose's letters changes; and he complains of the relentless sun, the stuffy atmosphere of the villa Sangalletti, and terrible headaches. Philip hears nothing at all from Easter to Whitsun of the following year; and, when a letter finally arrives in July, all is clearly not well. Ambrose writes of his illness and says that a friend and advisor of Rachel's called Rainaldi has recommended that Ambrose see a different doctor. Ambrose says he can trust no one and that Rachel watches him constantly.
Philip discusses the contents of the letter with his godfather, who thinks Ambrose may be having a breakdown or is suffering from a brain tumour. Nick tells Philip that Ambrose's father had died of a brain tumour, having suffered terrible headaches and other symptoms similar to those affecting Ambrose.
When Philip arrives at the villa Sangalletti, he is told by a servant that Ambrose is dead and that Rachel has left the villa. Philip is devastated by the realisation that he will never see Ambrose again. He calls on Rainaldi, who seems startled to see Philip but quickly recovers himself and explains that Ambrose's condition had deteriorated quickly, with his behaviour becoming very strange and that this was because of pressure on his brain. Philip feels intense dislike and mistrust for Rainaldi and Rachel.
Once back in Cornwall, Philip tries to overcome his sadness. Ambrose had appointed Philip's godfather to be his guardian until his coming of age, when he turned 25. Nick tells Philip that he has received a communication from Rainaldi, containing two pieces of information; the death certificate confirms that Ambrose's cause of death was a brain tumour and Ambrose had never changed his will in Rachel's favour so Philip is still heir to the estate. Nick accepts Rainaldi's word about Ambrose's death, but Philip is convinced that Rainaldi cannot be trusted; and, coupled with Rachel's abrupt disappearance from the villa, he is sure something is amiss.
Two weeks later, Nick receives word from Rachel saying that she has arrived by boat at Plymouth. She says that she has brought all Ambrose's possessions with her and wants to return them to Philip. Philip invites her to stay with him, even though he can hardly bear to think of her. Seecombe prepares the house for a female visitor; and, on the day Rachel is due to arrive, Philip goes out so that there will be no one to receive her.
Later, Rachel sends a note down, asking Philip to join her in her room. Rachel seems startled when she first sees Philip, and he is dumbfounded by the vision in front of him. Rachel is small and pale with dark hair and tiny hands, and she is dressed all in black. She bears no resemblance to the woman Philip has imagined since he first heard of her. Rachel thanks Philip for letting her visit and says that her arrival at the house was just as she had imagined it would be, because of all that Ambrose had told her about his Cornish home. Rachel quickly relaxes in Philip's company; and, despite his preconceived dislike for her, Philip finds himself at ease with her; all thoughts of anger, hatred and fear disappear as he gets to know and like her. The next day, Philip shows Rachel around the estate and introduces her to the tenants in the surrounding farms.
After a day or two, Philip realises that Rachel does not know about his trip to Florence, so he approaches the subject with her and shows her the last two letters that Ambrose had sent, explaining that it was the letters that prompted his journey. Rachel knows that the letters must have made Philip hate her; and he admits that he only invited her to stay so that he could accuse her of harming Ambrose, make her suffer, and then send her away. He goes on to tell her, that now they have met, she is so different from what he expected that he cannot hate her. She explains about Ambrose's illness; and, because they both loved Ambrose, a harmony develops between them.
Philip and Rachel adopt a routine which includes church on Sundays, followed by lunch with Nick and Louise Kendall, and the Reverend Pascoe and his wife and daughters. Rachel has brought many plants from Florence, and she spends lots of time working in the gardens with Tamlyn, the head gardener. Evidently, Rachel is an expert on plants and herbs and often provides advice on the use of herbal remedies to the staff in the house and the families in the nearby farms. She also brews tisane saying that it is better for you than tea. It is not long before Rachel is accepted as mistress of the household. From time to time, Rachel refers to Louise as Philip's future bride, which totally baffles him as he has no intention of marrying anyone.
Philip realises that, as Ambrose's widow and with no will to make provision for her, Rachel has no income; and he arranges for a quarterly sum to be paid to her. When Philip tells Rachel about the allowance, they argue, but then she agrees to accept the money and they are reconciled. Later, Philip sees a letter, addressed by Rachel to Rainaldi, in the postbag and feels disturbed by it.
One day, bad weather keeps Philip and Rachel indoors and they decide to unpack the things belonging to Ambrose, which Rachel had brought back from Florence. The cases contain all manner of things including clothes and books. Philip decides that he will distribute Ambrose's clothes to the tenants at Christmastime. When they begin to unpack the books, a note falls out of one of them. Philip reads it and then throws it onto the fire. The note included the following words, "This much I know, dear boy, that I cannot any longer, nay I dare not, let her have command over my purse, or I shall be ruined, and the estate will suffer."
Rachel sees the note, but Philip will not discuss it with her; and there is a feeling of constraint between them. Later, Rachel presses Philip to tell her what the note said; and he answers in vague terms saying that Ambrose had mentioned concerns about expenditure. Rachel tells Philip that Ambrose had always been generous with money until he became ill and then everything had changed and he had constantly questioned her about what she wanted the money for. She had even been obliged to ask Rainaldi to give her money so that she could pay the servants. She tells Philip that Ambrose did not like Rainaldi.
Winter approaches, and the household routine continues. Philip is totally happy; his only fear is that Rachel may want to leave one day, but she is busy with Tamlyn, planning the terraced gardens. Philip decides to hold a Christmas party for the tenants, and Rachel throws herself into the preparations. He wants to give Rachel something special for Christmas, so he decides to go to the bank to choose something from the family jewels. The most beautiful item is a collar of pearls, traditionally worn by Ashley brides on their wedding day. Philip brings the pearls home and gives them to Rachel just before the party on Christmas Eve. She is radiant with happiness and kisses him.
Everyone has a wonderful time at the party, which is attended by Nick, Louise and the Pascoes, as well as all the tenants. Back in the drawing room after the party, Nick speaks to Philip. He has a number of concerns about Rachel. Firstly, she is hugely overdrawn on her allowance. Secondly, Nick has heard a rumour that Rachel had a reputation for living a loose and extravagant lifestyle and people had been concerned when she married Ambrose in case she ran through all his money. Thirdly, Nick says Philip had no right to give Rachel the pearl collar; and, acting as his guardian, Nick says Philip must ask for it back. Rachel enters the room at that moment and hands the pearls to Nick. Philip is devastated but, when everyone else has gone, Rachel takes him in her arms and says he must not mind. But they are at cross purposes. Rachel loved wearing the pearl collar that she would have worn if she had married Ambrose in Cornwall and does not realise that Philip gave it to her because of the feelings he is developing for her.
Following the incident with the pearls, there is a coolness between Phillip and Nick. One day, Sam Bate, the tenant from East Lodge, asks to see Philip. Sam has found a letter, in Ambrose's writing and addressed to Philip, in the pocket of the jacket Philip gave him at Christmastime. Philip walks up to the top of the hill and sits down to read the letter. Ambrose had written the letter three months before he died and in it he tells Philip about his illness which takes the form of fever, headaches and strange moods. He talks of Rachel's recklessness with money and her habit of turning to Rainaldi, rather than himself. He says that Rainaldi has questioned him about his will and about providing for Rachel and that Rachel is always watching him. Finally, he says that, since his last bout of illness, he wonders if they are trying to poison him; and he asks Philip to go to him. Philip buries the letter up on the hill.
Philip does not mention the letter to Rachel, but he does talk to her about how different things would have been if Ambrose had left the estate to her. Rachel goes to the drawer and brings out Ambrose's unsigned will. Philip reads the will, which is in Ambrose's handwriting, and states that he leaves his property to Rachel, for her lifetime, passing at her death to the eldest of any children that might be born to both of them and, failing the birth of children, then to Philip, with the proviso that Philip should have the running of the property while Rachel should live. Rachel says that she does not know why Ambrose did not sign the will and that he had changed so much once he became ill, suspecting her of everything and not letting Rainaldi come to the house. The next day, Philip goes to Bodmin to see an attorney and gets a legal document drawn up, so that, on the day that he becomes twenty-five and inherits the estate, he can give it to Rachel and fulfil Ambrose's wishes. On his return home, Philip finds that Rainaldi has arrived. He stays for a week; Philip dislikes him and is jealous of the amount of time Rachel and Rainaldi spend together.
Philip becomes increasingly excited as his birthday approaches. On the day before his birthday he goes to the bank and withdraws all the family jewels. Later that day he goes to see Nick and shows him the unsigned will and the document that the attorney has drawn up. Reluctantly Nick agrees to witness Philip's signature on the document which gives the estate to Rachel, during her lifetime, providing she does not remarry. As midnight approaches Philip returns climbs up to Rachel room. He showers her with all the family jewels and afterwards they make love. Philip is so naïve that he believes this means that Rachel will marry him; however, she sees it very differently and was simply thanking him for the jewels.
The next morning Philip makes sure that the legal document giving Rachel the estate goes up to her room on her breakfast tray. He bursts into her room, but she sends him away, and later she goes out in the carriage. When she eventually returns, she says she has been to see Nick to clarify certain things in the document. She says that she has also seen Louise and repeats her thoughts on what a suitable wife Louise would be for Philip. Philip is stunned, he cannot understand what is wrong with Rachel, surely they are in love with each other and getting married? He tries unsuccessfully to recapture their mood of the night before. After dinner, Nick and Louise call in to have a birthday drink with Philip. Philip cannot contain himself and bursts out with the news that he and Rachel are to be married. There is a terrible silence and then Rachel apologises for Philip's ridiculous outburst. Nick and Louise leave. Philip has given Rachel his property, his money and the jewels. He has nothing else to give. He puts his hands around her neck and asks her to swear that she will never leave him, but his pressure on her throat prevents her from answering. He releases her, and she slips away to her room.
The next morning Philip sends a note to Louise, asking her to meet him by the church in the town. Although Louise is concerned about the events of the previous evening, she does not attempt to comfort Philip. Instead, she tells him that she believes that Rachel came to Cornwall with the specific intention of getting the Ashley money and that Philip has played into her hands, by misinterpreting her behaviour towards him. Louise does not want to hurt Philip, but she does want him to realise the truth so that he can begin to get his life back together. Philip stays in the town all day. It is a cold, rainy day; and the wind is blowing. By the time he returns, home he is chilled and wet. He finds that Rachel has invited Mary Pascoe, one of the vicar's daughters, to stay as her companion, making it impossible for Philip to talk to Rachel. That evening, Philip feels cold and unwell; and, by the morning, he is suffering from a stiff neck and terrible pains in his head. He collapses and has to be carried to his room. Philip is ill for many weeks; and, when he begins to recover, Rachel tells him he has had meningitis. He discovers that Rachel has been nursing him, and in his confusion he thinks they are married.
When he is well enough to go outside, he finds that the terraced gardens are complete and that work has begun on a sunken garden. Flowers are in bloom including the laburnum trees, which remind Philip of the laburnums that he saw growing beside Rachel's house in Florence. Rachel tells Philip that she will stay with him until he is well again and then she must go back to Florence. Philip asks Rachel to tell people that they are married; and, when she tells him that they are not, he collapses into sobs, his dream broken. Rachel agrees to stay a little longer, and Philip recovers to a certain extent, but he remains weak and has recurring headaches.
One day, Philip discovers that Rainaldi is staying at the Rose and Crown, beside the church in town, and that Rachel visits him there. When Philip challenges Rachel about this, she says that she did not tell him because he does not like Rainaldi. They argue and eventually Philip says that Rainaldi should come to the house to see Rachel. When Rainaldi arrives, Philip leaves him talking to Rachel and goes to his room. That night, Philip's fever returns; and Rachel resumes her role as nurse. Philip tells Rachel that she need not nurse him if she wants to spend her time with Rainaldi, but Rachel says that Rainaldi has gone. Thoughts of Rachel and Rainaldi make Philip remember the letter that Sam Bate gave him. As soon as he is well enough, he goes up the hill to where he buried the letter. He reads it again, especially the end part where Ambrose wonders if Rachel and Rainaldi are trying to poison him.
When Philip returns home, he notices that a letter has come from Rainaldi; but, later when he looks for it, he finds instead an envelope containing laburnum seeds. Philip thinks laburnum seeds are poisonous, and he begins to piece together a number of things– Rachel's relationships with Rainaldi, Ambrose and himself, her herbal remedies and tisane, Ambrose's illness, his own illness.
The next day is Sunday, and the foreman responsible for the work in the gardens has a word with Philip, telling him that the bridge over the sunken garden is only a framework and will not bear any weight. After church, Philip invites the Kendalls and the Pascoes to lunch. Philip asks Louise to stay behind when her father goes home. After Nick and the Pascoes have left, Rachel prepares some tisane for Philip, Louise and herself but Philip refuses to drink his.
Rachel goes out for a walk; and, as soon as she has gone, Philip asks Louise to help him find some sort of proof that Rachel is trying to poison him. As they search Rachel's room, Philip is surprised to find a letter from the bank thanking Rachel for returning the jewels. They can find nothing to incriminate Rachel and begin to wonder if they are misjudging her. Meanwhile, Rachel has walked to the terraced garden and stepped onto the bridge over the sunken garden. Philip finds her broken body lying amongst the timber and stone. He takes her in his arms, and she looks at him, calling him Ambrose before she dies.
The book's title reflects Philip's consistent references to Rachel as "my cousin Rachel" right up to the moment he realises he is in love with her.