In The Mousetrap, the action occurs at Monkswell Manor, where Mollie and Giles Ralston are preparing for the arrival of their dinner guests, fretting over the effect the blizzard raising outside will have on their plans, and barely noticing the news reports that there has been a murder in the area. The guests arrive—including a Mr. Paravicini, who was not invited but who seeks refuge after car trouble in the blizzard.
The next day, a police officer, Sergeant Trotter, arrives on skis and reveals a series of connections between the murder and Monkswell Manor. Suspicions abound and tensions rise until Mrs. Boyle is found strangled to death.
Sergeant Trotter assembles everyone for questioning and nobody manages to offer a truly satisfactory account of their innocence, leading to more suspicion and accusations amidst the growing realization that the murderer is among them.
At this point, I would urge you to leave it to your own conscience to decide how much you divulge regarding the climax of the play and the identity of the murderer. Traditionally, audience members are asked at the conclusion of The Mousetrap not to reveal the details of the play's ending to anyone outside the theatre. The way in which the play toys with the traditional whodunnit model is singularly famous in theatrical circles, and the tradition around not revealing the murderer's identity is akin to, though perhaps not as powerful as, the tradition that dictates actors never speak the name of "the Scottish play." Therefore, it remains at the writer's discretion as to how much they reveal regarding the ending of The Mousetrap.