Luke Fitzwilliam, a retired police officer from the Mayang Straits, a colonial posting in Southeast Asia, returns to England and accidentally exits his London-bound train at a junction station. While waiting for the next service, he shares a compartment with Miss Lavinia Pinkerton, a chatty elderly woman from Wychwood-under-Ashe. Miss Pinkerton confides she is traveling to Scotland Yard to report a series of murders in her village, naming several victims and predicting that Dr. Humbleby will be next. She insists the murderer is the last person anyone would suspect and that killing is easy so long as no one suspects you. Luke dismisses her as a harmless old lady.
The next morning, Luke reads that Miss Pinkerton was killed in a hit-and-run the same afternoon they parted. Over a week later, Dr. Humbleby's obituary appears: death by sudden septicaemia in Wychwood-under-Ashe. Luke's friend Jimmy Lorrimer devises a cover story: Luke will pose as a cousin of Jimmy's cousin Bridget Conway, who is secretary and fiancée to the press baron Lord Whitfield in Wychwood, and will claim to be writing a book on English folklore.
At Ashe Manor, Luke meets Bridget, a tall, dark-haired woman whose sharp intelligence unsettles him. Lord Whitfield is a small, pompous, self-made man who dismisses the late Humbleby as a fool. Bridget notes there have been "a lot of deaths in the last year" (33). Under the guise of folklore research, Luke pieces together the recent deaths: Amy Gibbs, a housemaid, apparently drank hat paint instead of cough mixture; Tommy Pierce, a boy, fell from a window at the library; Harry Carter, a pub landlord, reportedly fell from a footbridge. Luke also learns that Dr. Thomas, Humbleby's younger partner, benefited from Humbleby's death and is secretly courting Rose Humbleby, the dead doctor's daughter.
Luke and Bridget visit Miss Honoria Waynflete, a sharp spinster in whose house Amy died. Miss Waynflete hints at suspicions about Amy's death but refuses to speak plainly. Luke notices she keeps Wonky Pooh, Miss Pinkerton's Persian cat, who has an infected ear. He also catches an unsettling look on Miss Waynflete's face as she watches Bridget. They visit Mr. Ellsworthy, an affected antique dealer rumored to hold occult rituals, and encounter Major Horton, whose domineering wife died about a year earlier of acute gastritis.
Bridget confronts Luke, having seen through his cover at once. She points out that no modern girl paints hats and that a redheaded girl like Amy would never own red hat paint, meaning the substitution was staged. Luke tells her about Miss Pinkerton's warning, and they agree to investigate together.
Luke investigates the village's leading men as suspects. Dr. Thomas remarks that getting away with murder is "quite easy" (85). The solicitor Mr. Abbot quarreled fiercely with both Humbleby and Carter before each died. Rose Humbleby mentions that Miss Pinkerton was worried about Bridget. At a tennis party, Luke declares his love for Bridget, but they part on unresolved terms. Dr. Humbleby's widow, Mrs. Humbleby, tells Luke that "the world is a very wicked place" (131). Luke presses Miss Waynflete for the name she suspects, but she refuses, citing the risk of wrongly accusing someone of high standing. She reveals she was once engaged to Lord Whitfield, then a bootmaker's son named Gordon Ragg. Luke deduces her reluctance rules out Ellsworthy and Horton, leaving Thomas or Abbot.
On Midsummer Eve, Luke breaks into Ellsworthy's empty house and finds a notebook entry about Tommy Pierce and a sketch of Amy with a red cross slashed across her face. When Ellsworthy returns, Luke watches from hiding as the man prances down the hall with blood on his hands. Outside the Manor, Bridget tells Luke she has decided to break her engagement and marry him. They then discover the body of Rivers, Lord Whitfield's recently fired chauffeur, beneath a fallen stone pineapple from the gatepost. Sand grains in the wound indicate Rivers was struck by a sandbag and the ornament staged to look accidental. Luke is certain Ellsworthy committed this murder.
That evening, Lord Whitfield reframes the case. At dinner, he calmly lists people who defied him and died: Mrs. Horton, Tommy Pierce, Carter, Amy Gibbs, Humbleby, and Rivers. "They all die" (193), he says with serene satisfaction. Details snap into place: Whitfield sent grapes to Mrs. Horton during her illness, arranged for Tommy to clean windows at the library, and visited a research institute handling germ cultures before Humbleby's death. Luke concludes that Lord Whitfield is the killer.
In London, Sir William Ossington, now Assistant Commissioner of Police, agrees to investigate. A witness reported Whitfield's Rolls-Royce at the hit-and-run, but the police dismissed the report. Superintendent Battle is assigned to the case. Back in Wychwood, Miss Waynflete reveals why she ended her engagement: young Gordon snatched her canary and wrung its neck, and she saw he enjoyed it. She begs Luke to get Bridget away.
Bridget, however, has already told Lord Whitfield she intends to marry Luke, feeling it is the decent thing to do. Whitfield reacts with eerie calm, fondling a sharp Moorish knife. Luke drives Bridget from the Manor and tells her Whitfield is the murderer. Incredulous but gradually persuaded, Bridget insists on staying in Wychwood to help and accepts Miss Waynflete's invitation to stay at her house.
Superintendent Battle assures Luke that Bridget will be watched. Then Mrs. Humbleby warns Luke about Honoria Waynflete, calling her "a very wicked woman" (241), and recalls that her husband came home from Miss Waynflete's house with a bandaged hand. Luke realizes that Miss Pinkerton never said the killer was a man; he assumed it. He recalls Miss Pinkerton's face mimicking the murderous look she described and recognizes it as the same expression he noticed on Miss Waynflete's face when she watched Bridget. Frantic, Luke races toward Miss Waynflete's house.
Bridget is in danger. Miss Waynflete laced her tea with a sedative, but Bridget secretly poured it out. On a walk through isolated fields, Bridget pretends drowsiness while the truth crystallizes: Miss Waynflete is the killer, motivated by lifelong hatred of Whitfield for jilting her. She committed every murder and fed Gordon the delusion that Providence struck down his enemies, intending to frame him. Believing Bridget helpless, Miss Waynflete confesses. She wrung the canary's neck herself. She poisoned Mrs. Horton, swapped Amy's cough mixture for hat paint, pushed Carter off the footbridge, pushed Tommy from the windowsill, infected Humbleby's wound with discharge from Wonky Pooh's septic ear, pushed Miss Pinkerton into traffic and reported Whitfield's car number to police, and killed Rivers. She produces the Moorish knife bearing Whitfield's fingerprints and moves to cut Bridget's throat.
Bridget springs at her and they fight. With extraordinary strength, Miss Waynflete nearly overpowers Bridget, closing her hands around Bridget's throat. Luke, guided by screams, bursts through the trees and tears Miss Waynflete away just in time.
Battle explains that Miss Waynflete had a hereditary predisposition to a mental health condition, worsened by thwarted ambition and rejection. Lord Whitfield reveals the truth: Honoria wrung the canary's neck, and he broke off the engagement because the act horrified him. Bridget explains she deduced this because she knew Gordon was too tenderhearted to harm a bird, so Miss Waynflete's version had to be a lie. Ellsworthy's blood on Midsummer Eve is explained by a sacrificed cockerel. Luke and Bridget, shaken but united, resolve to leave death behind and begin to live.