Sister

The novel is narrated entirely as a letter from Beatrice (Bee) Hemming to her younger sister Tess, who is dead, recounting the events surrounding Tess's disappearance and murder. Interwoven with this letter is what appears to be Bee giving her witness statement to Mr. Wright, a kind, bereaved lawyer at the Crown Prosecution Service (CPS), in preparation for a criminal trial.

Bee lives in New York with her fiancé Todd in a relationship she later recognizes as superficial and passionless. On January 26, her mother phones to say that Tess, an art student in London, has been missing for four days. Bee flies to London immediately and meets Detective Sergeant (DS) Finborough, a thoughtful detective. She reveals that Tess is more than eight months pregnant by Emilio Codi, a married tutor at her art college who forced Tess to take leave when the pregnancy became visible. Tess's elderly landlord, Amias Thornton, reported her missing when she failed to keep a supper date, mentioning nuisance phone calls.

DS Finborough arranges a televised reconstruction of Tess's last known movements, with Bee volunteering to play her sister. Afterward, Bee visits Tess's basement flat and finds her toiletries still in the bathroom, destroying any hope that Tess left voluntarily. She also meets Kasia Lewski, Tess's pregnant Polish friend, who comes looking for Tess and mentions they met at a prenatal clinic.

Tess's pregnancy carries special weight because of family history. Their brother Leo died of cystic fibrosis (CF), a genetic lung disease, at age eight. At 12 weeks, Tess's baby was diagnosed with CF through genetic screening. A company called Chrom-Med offered a gene therapy using an artificial chromosome invented by Professor Rosen to replace the faulty gene in utero. The therapy appeared to work, and Tess named her cured baby Xavier, after Leo's middle name.

DS Finborough brings news that Tess already gave birth. St. Anne's Hospital confirms she delivered on January 21 and discharged herself the same day. Xavier was stillborn. Tess was referred to psychiatrist Dr. Hugo Nichols, who initially diagnosed postpartum depression but revised the diagnosis to postpartum psychosis after Tess's death. Bee is briefly relieved but senses the explanation does not fit.

Simon Greenly, a fellow art student in love with Tess and the son of a Member of Parliament, reveals he was with her in Hyde Park the day she disappeared. Tess was terrified, said a man had followed her, and begged Simon for protection. He confessed his love instead; Tess rejected him, and he left her alone. Bee calls the police, who search the park and find Tess's body in a derelict Victorian toilets building. She has been dead for five days, her wrists slashed. The postmortem rules suicide. Bee refuses to accept this: After watching Leo fight for every breath, she insists Tess valued life too deeply to end it.

Bee investigates on her own. She confronts Emilio, but his wife provides an alibi. She visits Dr. Nichols, who saw Tess only once through the National Health Service (NHS), Britain's public healthcare system, and interpreted Tess's reports of threatening phone calls as hallucinations. He suggests Bee look for paintings. She finds four nightmarish canvases hidden among Tess's work depicting a screaming woman and a masked man.

While sorting Tess's mail, Bee discovers a phone bill showing Tess called her 15 times in the days before her death. Tess also called the day Xavier was born, but Bee missed the call while celebrating a promotion and then left for a remote cabin with no reception. The guilt is devastating, but the calls reinforce Bee's conviction that Tess did not kill herself.

Bee's school friend Christina Settle, now a doctor, helps interpret the postmortem blood tests. Three drugs were found in Tess's body: one to stop breast milk production, a large dose of sedative, and phencyclidine (PCP), a powerful hallucinogenic that causes terrifying visions. The sedative explains why Tess could not resist her attacker, and the PCP accounts for the hallucinations Dr. Nichols attributed to psychosis. On Tess's answering machine, Bee finds a lullaby recorded the evening after Xavier's death, evidence that someone deliberately tormented a grieving mother. DS Finborough reveals police already knew about the drugs but dismissed them, and his superior, Detective Inspector Haines, dismisses the lullaby.

The case shifts when Emilio reveals he is not a CF carrier. Since both parents must carry the gene for a baby to have CF, Bee concludes Xavier never had the disease. She theorizes the trial treats healthy babies to inflate its success rate before Chrom-Med's stock market flotation. Kasia's boyfriend Mitch is also found not to be a CF carrier, confirming the pattern. A third woman from the same prenatal clinic, Hattie Sim, reveals her baby also died, she was forced to have a hysterectomy, and the doctor who treated her always wore a mask. All three women's hospital notes have vanished.

Bee's investigation destroys her relationship with Todd, who believes Tess committed suicide. She breaks off the engagement. Kasia, beaten by Mitch, arrives at Bee's door that night, and Bee takes her in. Tess and Xavier are buried together at the family churchyard next to Leo. The sisters' estranged father arrives unexpectedly and embraces Bee and their mother for the first time.

Professor Rosen contacts Bee with alarming news: An active virus vector—a biological carrier used to deliver genes into cells, and one his legitimate CF trial does not use—has been ordered under his CF trial number at St. Anne's. He concludes someone has hijacked his trial to test an illegal gene but refuses to involve the police, fearing it will destroy his legitimate CF cure. Dr. William Saunders, an obstetrician at St. Anne's who has offered Bee comfort, volunteers to identify who runs the trial. He returns blaming Dr. Nichols, who was previously a genetics researcher, then appears to phone the police. He and Bee walk to Hyde Park to lay daffodils at the toilets building.

William bought daffodils because, he claims, Bee told him they were Tess's favorite flower. But Bee never told him this. When she confronts him, he pushes her inside and holds a knife to her throat. William confesses that he, not Dr. Nichols, discovered an IQ gene and hijacked the CF trial to test it on humans. His gene impaired the babies' lung function, killing them at birth. He killed Tess because the gene had entered her ovaries, meaning every future pregnancy would produce evidence of his crime. William binds and gags Bee, forces sleeping pills into her mouth, and stages a copycat suicide. While he reads aloud the letter Tess was writing before her murder, Bee spits most of the pills into her coat. William's pager sounds and he leaves.

In the novel's final revelation, Bee discloses that Mr. Wright and the CPS offices were imagined. She has been lying bound and gagged on the concrete floor of the toilets building all along, constructing the witness statement to survive the night. The blackouts and nausea she described were real symptoms of the sedative. Throughout the night, Kasia pages Bee every 20 minutes. When Bee does not come home, Kasia goes to the police. As dawn breaks, Bee hears voices shouting her name and finally Kasia screaming "Beata!" A siren approaches. The door opens. The novel ends with Bee's final words to Tess: "I'm sorry. I love you. I always will."