Jerry and Joanna Burton, brother and sister from London society, take a country house in idyllic Lymstock so that Jerry can rest from injuries received in a wartime plane crash. They are just getting to know the town's strange cast of characters when an anonymous letter arrives, rudely accusing the two of not being brother and sister, but lovers. They quickly discover that these letters have been recently circulating around town, indiscriminate and completely inaccurate.

Things flare up when Mrs. Symmington, the wife of the local solicitor, commits suicide upon receiving a letter which states that her second child is actually illegitimate. Her body is discovered with the letter, a glass containing potassium cyanide and a torn suicide note which reads "I can't go on".

An inquest is held and the verdict of suicide is brought in. The police begin to search for the anonymous letter writer. The Burtons' maid, Partridge, receives a call from the Symmington's maidservant, Agnes, who seems distraught over something. Partridge asks Agnes over to tea the next afternoon, but Agnes never arrives. Agnes' body is discovered in the closet the next day by Mr. Symmington's rather shabby-dressed step-daughter, Megan.

Scotland Yard sends someone to investigate, and comes to the conclusion that the letter-writer/murderer is a middle-aged woman who must be one of the prominent citizens of Lymstock. Progress is slow until the vicar's wife calls up an expert of her own, Miss Marple. Jerry Burton gives Miss Marple some vital clues by telling her of the contents of his dreams and his disconnected thoughts.

There is a break in the case when the Symmington's beautiful young governess, Elsie Holland, receives an anonymous letter typed on the same typewriter, proven to have been used to create envelopes for all the previous letters. The doctor's sister, Aimee Griffith, is arrested, since she has been seen both typing the letter and delivering it.

On the way to London for a visit to his doctor, Jerry takes Megan along with him to London where he buys her some new clothes to make her look presentable. He also begins to realize that he has fallen in love with her. When they return to Lymstock, Jerry asks Megan to marry him, and she refuses. As a result, Jerry goes to Mr. Symmington to ask for his permission and to inform him of her refusal. Symmington, who is eager to have Megan off his hands, tells Jerry that he will speak with her.

Later that evening, Megan goes to Mr. Symmington’s office, and tries to blackmail him. He coolly pays her, but later, when Megan is asleep he tries to murder her by putting her head in the gas stove. He is immediately stopped by Jerry and the police. It is revealed that Miss Marple wished to prove Mr. Symmington’s guilt in this way, and that Megan was brave enough to assist her.

Symmington had written all the letters as a cover-up for killing his wife. He had used phrases from a similar incident, done by a school-girl, which fooled the police into thinking that a woman had been the letter writer. He murdered his wife by the use of cyanide and then planted the letter and a fake suicide note to disguise his crime. He committed the murders because he wished to marry his children's governess. Aimee Griffith, who was in love with Symmington, had written only the letter to Miss Holland, out of jealousy and to try to protect the man she loved from marrying the wrong woman.

Megan, in light of recent events, finally realizes that she does indeed love Jerry. His sister Joanna marries the local doctor, and both couples settle down in Lymstock instead of returning to London. After explaining the case, Miss Marple returns home to St. Mary Mead.

The book's title, The Moving Finger, is emphasized twice. The first is how the accusatory letters point blame from one town member to another, the second is from the addresses on the letters, which the Scotland Yard agent uses to determine the envelopes were all "typed by someone using one finger" in order to avoid a recognizable 'touch.'