The title is a quote from Hamlet (I.v.27-28), where the Ghost comments about his own death, "Murder most foul as in the best it is/But this most foul, strange and unnatural."
Margaret McGinty, a barmaid and former actress, is found hanged, and her lodger, Harold Taylor, caught at the scene, seems plainly guilty. Everyone believes it to be an open-and-shut case ... except for Miss Marple. She is the lone holdout in the jury that tries him, leading to a mistrial.
Despite the disapproval of Inspector Craddock, Miss Marple decides to delve into the case. She poses as a gatherer for a church 'jumble sale' to enter and search Mrs. McGinty's home. She finds a newspaper with words cut out and several programs for a murder mystery play recently performed in the town. These clues lead her to suspect Mrs. McGinty of blackmailing a member of the repertory company.
She auditions for the repertory theatre players, the Cosgood Players, under their actor/manager Driffold Cosgood (Ron Moody). Cosgood is unimpressed by her acting ability, but as she is willing to work for free and mentions she is independently wealthy, takes her on. Miss Marple knows she is on the right track when actor George Rowton (Maurice Good) is poisoned moments later. She secures accommodation in the boarding house in which the cast are staying to further her investigation and Cosgood leaves a copy of his play Remember September in her bedroom to read. Narrowly avoiding an attempt to silence her (one which claims the life of another actress by mistake), Miss Marple unmasks the killer. Cosgood appeals to her to finance Remember September, but she says "Mr Cosgood, whatever else I am, I am definitely no angel."
- Margaret Rutherford — Miss Jane Marple
- Ron Moody — H. Driffold Cosgood
- Charles Tingwell — Inspector Craddock
- Andrew Cruickshank — Justice Crosby (credited as "Andrew Cruikshank")
- Megs Jenkins — Gladys Thomas, Mrs McGinty's sister
- Dennis Price — Harris Tumbrill, a theatrical agent
- Ralph Michael — Ralph Summers, a philandering former matinee idol
- James Bolam — Bill Hanson, an actor
- Stringer Davis (Margaret Rutherford's real-life husband) — Jim Stringer
- Francesca Annis — Sheila Upward, an actress and heiress engaged to Bill Hanson
- Alison Seebohm — Eva McGonigall, a premonition-prone actress
- Terry Scott — Police Constable Wells
- Pauline Jameson — Maureen Summers, Ralph's wife
- Maurice Good — George Rowton
- Annette Kerr — Dorothy, an actress and murder victim
- Windsor Davies — Sergeant Brick
- Neil Stacy — Arthur (as Neil Stacey)
- Stella Tanner — Flory, the landlady
The theatre in which the Cosgood Players perform and where much of the action takes place was The Palace Theatre in Watford, Hertfordshire. At the time, it was being run by Jimmy Perry (creator of Dad's Army, It Ain't Half Hot Mum and Hi-De-Hi) and his wife Gilda.
The scene of the murder and associated village scenes were filmed in Sarratt, Hertfordshire.
The title is spoofed in the 1990s television comedy series Murder Most Horrid, starring Dawn French.