Agatha Christie's play The Mousetrap begins with the murder of Maureen Lyon. Then the scene shifts to Monkswell Manor, where a young couple, Giles and Mollie Ralston, welcomes their guests. Among these is a man who identifies himself as Sergeant Trotter. He is investigating Maureen Lyon's death, and he has come to Monkswell Manor because of a note found at the scene of the crime. Maureen Lyon, Trotter explains, may have been killed by the oldest of three foster children, the Corrigans, whom she and her husband were imprisoned for abusing. The youngest boy died in their care.
As the play progresses, one of the guests, Mrs. Boyle, is also murdered. She has been revealed to be one of the judges who originally placed the children with the Lyons. As the investigation continues, and the play draws to a close, Trotter calls on Mollie Ralston and identifies her as a schoolteacher who once taught the Corrigan children. The youngest boy wrote to her for help, but she never responded. Mollie admits the truth of it but says that she was ill when the letter came and never read it until it was too late.
At this point, Trotter surprisingly draws a gun on Mollie Ralston. He is Georgie Corrigan, the eldest of the abused children, and he is out for revenge. He has killed his foster mother and the judge who put him and his siblings into such a horrible situation. Now he wants to eliminate Mollie for not helping them. Another guest, Miss Casewell, runs in and calls Trotter by his real name, revealing herself to be his sister. Together with the guest Major Metcalf (who really is a policeman), they subdue Trotter/Corrigan.