When the widow, Mrs. Ferrars, commits suicide, and the wealthy Roger Ackroyd is murdered within a few days, the small English village of King's Abbot is even more full of gossip than usual. The murder is a complete mystery to the incompetent local police. Fortunately, though, the famous detective Hercule Poirot has just moved into King's Abbot and agrees to take on the case, even though he has already decided to retire from detective work.

Everyone in Mr. Ackroyd's household is a suspect. Poirot investigates the murder with Dr. James Sheppard, the town doctor and good friend of Mr. Ackroyd, by his side. As Poirot unravels the case, many secrets, largely around the themes of love and money, come out about each member of the household. Although every member of the household is a suspect at some point, the case looks the worst for Ralph Paton, Mr. Ackroyd's stepson. Ralph has several motives and many clues pointing straight at him, but this does not fool Poirot.

In a shocking twist at the end, Poirot discovers that James, the narrator of the story is the killer. James was blackmailing Mrs. Ferrars for money in return for keeping her secret about the murder of her husband. Mr. Ackroyd found out about the blackmail through a suicide letter Mrs. Ferrars wrote to him. James had to silence Mr. Ackroyd.

Poirot unravels the entire tale and then suggests to James a way out of prison and shame. The way is suicide. James decides to take Poirot's advice if only to save his beloved sister from the shame and disappointment of finding out that her brother is a murderer. First, James goes home to finish the manuscript he has been working on throughout the investigation, which consists of a written record of the case. He completes the manuscript by adding what he had previously omitted about his own guilt, sends the manuscript to Poirot and then kills himself.