The news on the radio at Monkswell Manor relates a murder that has recently taken place. Mollie and Giles Ralston, the young, newly-married owners of the once-regal estate which they recently converted into a guest house, hardly notice the news. They are far too busy preparing for the arriving of their first guests—and concerned that the blizzard raging outside may hamper their arrival.
Christopher Wren arrives first. He is an obviously neurotic young man who speaks to the Ralstons with a familiarity that makes them rather uncomfortable.
The next to arrive is Mrs. Boyle, a generally unpleasant person who is dissatisfied with just about everything and everyone.
Next comes Major Metcalf, a middle-aged man who is very military in manner and bearing.
Miss Casewell, a young woman who is just a bit masculine, is the next to arrive. She relates more details about the murder that recently took place.
Then an unexpected guest, Mr. Paravicini, arrives announcing that his car has overturned in a snowdrift. He is just happy to have found someplace to get in out of the weather.
The next day finds Mrs. Boyle generally getting on everyone’s nerves. Mollie announces that a phone call from the police has informed her that an officer is being sent to the manor, in spite of the weather. She was given no indication as to why the officer is coming. Several of the guests are obviously unnerved by the announcement. It becomes apparent that the Ralstons really don’t know much about their guests. Gradually, everyone is becoming a bit suspicious of everyone else.
Soon the police detective, Sergeant Trotter, arrives on skis. He relates that the murdered woman was once a resident of a nearby house. A few years back, the courts sent several children there for care and protection. The children had been terribly abused at the house, and one of them had died before the courts had the other children removed. The murdered woman was the one who had abused those children. A note had been left on the body claiming there were “three blind mice” who would be murdered, and the name Monkswell Manor was also on the note. Sergeant Trotter believes that someone at the manor had a connection of some sort to the murder victim. The murderer, he says, may well be among them even now. Each of the guests, as well as the Ralstons, denies having any knowledge of the situation whatsoever.
Mrs. Boyle, however, recalls privately with Major Metcalf that she was once a magistrate on the bench. In fact, she was the one who had sent those children to the place where they were so miserably abused. Suspicion at Monkswell Manor is growing rapidly, and the telephone no longer works.
Mollie hears a scuffle and a scream coming from the library and enters to find Mrs. Boyle has been strangled.
Sergeant Trotter assembles everyone for questioning. Each accounts for his or her whereabouts, but none satisfactorily. Soon, everyone becomes suspicious of everyone else. Giles accuses Christopher Wren of being the most likely to be the killer since he is about the same age the oldest of the remaining children would be now. Mollie points out that the killer may be the father of the abused children and therefore wouldn’t necessarily be a young person at all. Trotter casts suspicion on Giles Ralston, producing a London newspaper from the pocket of Giles’s overcoat. Trotter is also suspicious of Miss Casewell, who, in turn, sees something strange in his behavior. Everyone realizes a murderer is among them—and two more victims, the other bind mice, are in danger.
So, who is the next victim? Will the murderer be unmasked in time to stop more deaths? The Mousetrap has kept audiences guessing about these answers and many more for over five decades in this classic who-done-it by the master of British murder mysteries.