Arthur Hastings is on leave from World War I when he runs into an old friend, John Cavendish. After they’ve gotten reacquainted, John invites Hastings to spend time at his family’s country house, Styles Court. He explains that there’s some tension at Styles Court these days, since his stepmother, Emily, recently married a younger man named Alfred Inglethorp. Everyone is suspicious of Alfred, thinking he’s “fortune hunting.” When he first arrived, he claimed to be a distant cousin to Evelyn Howard, Emily’s closest friend. Emily welcomed Alfred into the home, and it wasn’t long before they announced their marriage.
Hastings’s first days at Styles Court mainly consist of meeting everyone who lives at the country house. He takes a particular interest in John’s wife, Mary, whom he finds attractive. He also enjoys talking to Cynthia Murdock, a young woman who mixes medicines at the nearby hospital and who has been living at Styles ever since she was orphaned. Like everyone else at Styles, he dislikes Alfred Inglethorp immediately after meeting him, finding himself greatly unsettled by the man’s presence.
It isn’t long before calamity breaks out. Hastings hears through Cynthia that Emily and Evelyn have had a terrible fight. Evelyn apparently spoke her mind to Emily, saying Alfred is just using her for her money and waiting for her to die. She also insisted that Alfred has been having an affair with the neighbor, Mrs. Raikes, but Emily refused to believe these allegations. Evelyn has therefore decided to leave Styles at once. As everyone watches her drive away, Hastings spots a man with a big dark beard walking toward the house; it’s Dr. Bauerstein, an expert in poison who’s visiting from London. Bauerstein and Mary Cavendish are close friends and spend a lot of time together.
The next day, Hastings and Lawrence—John’s younger brother—pay Cynthia a visit at the dispensary. Lawrence, who studied to be a doctor, opens the poison cabinet while looking for tea. He also stays behind in the dispensary while the others step out. He then goes with Hastings and Cynthia to the post office, and Hastings is astounded to run into his old friend, Hercule Poirot. Hastings insists that Poirot is an incredible detective, claiming that his friend is a celebrity of sorts. Because of this unexpected run-in, Hastings is in a good mood when he and the others return to Styles, so he’s caught off guard by the tense atmosphere in the house. He and Cynthia decide to play tennis, and once they meet on the courts, Cynthia tells Hastings what she has learned: Dorcas, the maid, told her that Emily and Alfred had a terrible fight.
That evening, everyone gathers for coffee, but Emily decides to have hers in her room because she has some letters to write. Alfred pours it for her and takes it up. Shortly thereafter, Dr. Bauerstein steps inside, since Alfred saw him walking by and encouraged him to come in for coffee. At the end of the evening, Alfred announces that he has business in town and won’t be back until late.
Later that night, Hastings wakes up to a terrible commotion. Awful sounds issue from Emily’s bedroom, but the doors are locked. Lawrence, Hastings, and John break open the door between Emily and Cynthia’s bedrooms only to find Emily in the throes of violent convulsions. It isn’t long before Dr. Bauerstein makes his way into the room, saying he was walking by at just that moment. But he’s too late, and Emily dies, saying “Alfred—Alfred—,” though she’s unable to finish her sentence. At that moment, everyone realizes Alfred isn’t in the house.
Emily’s regular doctor, Dr. Wilkins, comes to inspect Emily, and Dr. Bauerstein asks to have a word in private with him. Hastings senses that something is amiss, realizing that Bauerstein suspects that Emily was poisoned. Wanting to get ahead of the case, Hastings asks John if he can bring in Poirot to investigate, and though John is hesitant, he eventually agrees. Meanwhile, Wilkins and Bauerstein finish their conversation and inform John and Lawrence that, because of the strange circumstances of Emily’s death, they can’t issue a death certificate until there’s an official inquest (a court proceeding aimed at gathering information about an incident).
Hastings rushes to Poirot’s apartment and tells him everything. Poirot tells him to take his time—it’s important to approach such matters levelheadedly and methodically gather the necessary details. They then go to Styles, where Poirot conducts an investigation of Emily’s bedroom. He finds a piece of torn green fabric on the bolt between Cynthia and Emily’s rooms, a crushed coffee cup, a still-wet coffee stain and some candlewax on the rug, Emily’s dispatch case and its key, and an empty box of bromide powders, which Emily uses to help her sleep. He also roots around in the ashes of the fireplace and finds a scrap of heavy paper, which he thinks was part of a will.
After breakfast, Poirot, John, and Hastings meet with Emily’s lawyer, Mr. Wells, who tells them that Emily wrote to him the day before asking him to visit in the morning—perhaps to execute a new will. He also explains that before marrying Alfred, Emily’s most recent will stipulated that John would inherit her fortune. According to British law, though, this will was overridden when Emily married Alfred, though it’s unclear if she knew this. When Mr. Wells and John try to go through Emily’s papers, they discover that her dispatch case has been broken into—somebody forced the lock. Poirot is shocked when they tell him this, since he locked the case only an hour before, meaning that the murderer must have taken the risk of sneaking in and stealing something from the case while everyone was still in the house. Poirot is so excited by this development that he compulsively straightens the items on the mantelpiece in Emily’s bedroom, and Hastings notices that his hands are shaking violently.
At the inquest several days later, Alfred doesn’t provide a plausible alibi to prove his innocence. In fact, he does such a bad job of sounding innocent that it rankles Poirot, who pleads with an old colleague, Inspector Japp, not to arrest Alfred. Japp thinks he’s crazy, but he also respects Poirot, so he agrees to hear him out. Poirot then gathers everyone (including Japp) back at Styles, and he gets Alfred to confess to why he won’t say where he was on the afternoon that somebody who looked like him bought strychnine from Mr. Mace, a local pharmacist who testified at the inquest. The reason, Alfred admits, is that he was with Mrs. Raikes and didn’t want anyone to know. As such, Japp doesn’t arrest him.
Not long after this meeting, Hastings off-handedly mentions to Poirot that Dr. Bauerstein stopped by on the night of Emily’s death and had a cup of coffee. Poirot is beside himself—this detail changes everything. He borrows John’s car and drives to a nearby laboratory to have a sample of cocoa tested. Emily used to have cocoa each night, but Poirot discovered that a housekeeper named Annie—who always brought the cocoa upstairs—saw what she thought was salt on the saucer that night. She gave the cocoa to Emily anyway, and now Poirot wants to have it tested, even though Bauerstein supposedly already had this done.
In the coming days, Poirot and Hastings find a fake beard in a box of dress-up clothes in the attic at Styles—an important detail, since Poirot suspects that somebody impersonated Alfred when buying the strychnine from Mr. Mace. Around this time, Hastings grows frustrated with Poirot, who never fully explains his theories and is often quite secretive. Tired of the entire affair, Hastings goes for a walk in the woods and takes a nap. Upon awakening, he hears John and Mary arguing with each other nearby. John asks Mary to stop seeing Dr. Bauerstein, but she refuses, pointing out that John spends plenty of time with certain people Mary would rather he not see. She then leaves John in the woods, at which point Hastings emerges and acts like he didn’t overhear anything. Upset about Mary’s relationship with Dr. Bauerstein, Hastings forms a theory that Bauerstein is the killer, excitedly telling John what he thinks and insisting that Bauerstein poisoned Emily’s coffee when he first came into the house on the night of the murder.
But Hastings’s theory falls flat later that day when he goes to check in on Bauerstein and discovers he’s been arrested—but not for murdering Emily, Poirot later tells him. Rather, Bauerstein has been arrested for espionage; he’s a foreign spy, not a murderer. Poirot informs Hastings that Bauerstein didn’t even have feelings for Mary. Rather, he just wanted everyone to think they were having an affair, since the gossip would account for why he was always sneaking around at odd times. Later that same day, Hastings finds Poirot in a state of agitation, since he has figured out who the killer is but isn’t sure if he should say anything. After all, he says, a “woman’s happiness” is at stake, though he won’t elaborate. Upon returning to Styles that evening, they discover that John Cavendish has been arrested for the murder.
John stands trial for murder two months later. Mary rents a house in London so everyone can stay there while the hearing takes place. During the trial, John’s defense attorney implies that there’s just as much evidence to convict Lawrence as there is to convict John. Still, the prosecution makes a compelling case against John, revealing that Inspector Japp found a vial of strychnine in his bedroom—the same vial somebody apparently bought from Mr. Mace while disguised as Alfred. Also in John’s bedroom was a monocle similar to one Alfred wears. But John’s lawyer focuses on Lawrence’s suspicious behavior, pointing out that Evelyn Howard found a letter addressed to Lawrence from a famous costume company, ultimately suggesting that Lawrence ordered the fake beard and used it to buy strychnine while dressed as Alfred. There’s also the fact that Lawrence visited the dispensary the day before Emily’s death. He not only opened a cupboard containing strychnine, but also touched a bottle of it, according to a fingerprint analysis.
After the first day of the trial, Poirot is disturbed. He needs to find one last piece of evidence to prove his suspicions. Hastings notices that Poirot’s hands are shaking as he speaks, and he comments that he has only ever seen his friend’s hands shake once before: when he was straightening out the items on the mantelpiece after discovering that Emily’s dispatch case had been forced open by someone in the house. This memory triggers something in Poirot’s mind, and he excitedly runs away. The trial will resume on Monday, and Poirot doesn’t return until Sunday evening. Upon his return, he summons everyone (including Alfred, who’s staying in a separate apartment) to explain what he has found. He then reveals that he discovered a letter from Alfred to Evelyn Howard—a letter that told her not to worry about the delay in their plan and insisting that they would lead a life of happiness together once Emily was dead. The letter was in a vase on Emily’s mantelpiece; Poirot thought to look there because Hastings reminded him that he had straightened that vase after finding the dispatch case unlocked, which made him realize that he shouldn’t have needed to do this, since he had already straightened it when he first entered the room that morning. He thus knew to look inside the vase and ultimately found the letter, in which Alfred praises Evelyn’s idea to use bromide powders, though it’s unclear at first what this means. But Poirot explains exactly what happened: Alfred and Evelyn are secret lovers who poisoned Emily in the hopes of making off with her fortune. But they didn’t poison her coffee. Rather, they added her bromide powders to a medicinal tonic she was already taking on a doctor’s orders. The bromide caused the strychnine to crystalize at the bottom of the bottle, and Alfred made sure to carefully pour out the medicine each night for his wife so that her final dose would contain enough strychnine to kill her. Emily was supposed to take her final dose on the evening Evelyn left, but she ended up forgetting and taking it the next night. For this reason, Alfred wrote a letter to her assuring her that the murder would take place the following night, but he never had a chance to mail it.
According to Poirot, Emily couldn’t find any stamps on her last night amongst the living, so she forced open Alfred’s writing desk, where she found the incriminating letter to Evelyn. She immediately realized she had to write a new will that wouldn’t benefit Alfred, which is why she asked her lawyer to come the following day, since she didn’t suspect she would die that very night. As Poirot goes through what happened, Alfred lunges at him, but Poirot steps aside and lets him fall to the floor. Alfred and Evelyn are then arrested.
In the aftermath of the entire ordeal, Poirot answers some of Hastings’s lingering questions. Evelyn, he says, planted incriminating evidence against both John and Lawrence while everyone was busy suspecting Alfred of the crime. Poirot, for his part, knew from the beginning that Alfred was guilty, but he didn’t want him to get arrested right away, since there wasn’t enough evidence to convict him. Alfred knew this and was aware that nobody can be tried twice for the same crime in England. He therefore wanted to be arrested before the case against him was very strong. But Poirot stopped that. Of course, Poirot also knew that John Cavendish wasn’t guilty, but he didn’t say anything because he wanted to help him and Mary—he knew they both loved each other despite the tension in their relationship, and he was confident that going through the hardship of a murder trial would bring them together. He was right: John and Mary are deeply in love once again. As for Hastings, he’s disappointed that he doesn’t have a lover, but Poirot tells him not to despair, suggesting that there will be new opportunities to meet women whenever they investigate their next case together.
