Plot
The family of wealthy mystery novelist Harlan Thrombey attends his birthday party at his estate. The next morning, Harlan's housekeeper Fran discovers him dead with a slit throat. Police detectives Lieutenant Elliot and Trooper Wagner believe Harlan died by suicide, but private detective Benoit Blanc is anonymously hired to investigate Harlan's death. Blanc learns Harlan had strained relationships with his family members, giving several of them plausible motives for murder.
Unbeknownst to Blanc, Harlan's nurse Marta Cabrera believes she injected Harlan with a lethal dose of morphine after mixing up his bedtime medications the night of the party. To protect Marta from being blamed for his death, Harlan instructed Marta to create a false alibi before he slit his own throat: she was to be seen leaving the house, sneak back in through a window, and disguise herself as Harlan in order to make it appear that he was alive after she left for the night. Marta cannot lie without vomiting, so she gives accurate but incomplete answers when questioned. She agrees to assist Blanc's investigation and conceals evidence incriminating her. At the reading of Harlan's will, Marta is bequeathed his entire fortune and property, stunning the Thrombeys. Harlan's grandson Ransom Drysdale helps Marta flee but manipulates her into confessing to him. Ransom offers further assistance in exchange for a portion of Marta's inheritance. Meanwhile, the other Thrombeys unsuccessfully attempt to influence Marta to renounce the inheritance, even threatening to have her undocumented mother deported.
Marta receives a blackmail note containing a partial photocopy of Harlan's toxicology report. She and Ransom drive to the medical examiner's office, only to find it burned down. Marta receives an email proposing a meeting with the blackmailer; Blanc and the police spot them, leading to Ransom being arrested. At the meeting, Marta finds that Fran, the blackmailer, has been drugged; she performs CPR on Fran and calls an ambulance. Marta confesses to Blanc, but discovers Ransom has already implicated her. Out of moral obligation, Marta believes she must confess to the Thrombeys, which would invalidate the bequest under the slayer rule.
At the mansion, Marta finds Fran's copy of the full toxicology report, which shows Harlan had only trace amounts of morphine in his blood. Blanc reveals his deductions to the police, Marta, and Ransom. Blanc deduces Harlan told Ransom about his will, prompting Ransom to swap Harlan's medicines to cause Marta to kill him unknowingly. However, Marta actually gave Harlan the correct medication, recognizing it by viscosity without reading the label due to her experience as a nurse; she only believed she had poisoned Harlan after reading the label on the bottle with the switched content. When the death was reported as a suicide, Ransom anonymously hired Blanc to entrap Marta. Fran saw Ransom tampering with the crime scene to remove the switched medications, and sent him the blackmail note. After Ransom realized that Marta was not responsible for Harlan's death but believed she was, he forwarded the blackmail letter to Marta and burned down the medical examiner's office to destroy evidence of her innocence. Ransom then overdosed Fran with morphine, intending for Marta to be caught with Fran's corpse.
Marta receives a call from the hospital informing her that Fran has died from her overdose. Since there is no evidence of Ransom committing a murder, Marta lies to the group that Fran is alive while suppressing her urge to vomit. This tricks Ransom into confirming his involvement with Fran's overdose, inadvertently confessing to murder. Realizing his loss, Ransom attacks Marta with a knife from Harlan's collection but the knife is a harmless retractable stage knife. The police promptly arrest him.
Blanc tells Marta he suspected early on that she played a part in Harlan's death, noting a drop of blood on her shoe. He tells Marta her innocence prevailed because she made ethical choices that obstructed Ransom's attempts to incriminate her. As Ransom is taken into custody and the rest of the family is gathered outside in defeat, Marta watches from the balcony of what is now her mansion, sipping from Harlan's coffee mug that reads "My House, My Rules, My Coffee!!".
