In a surprise ending to the play, the murderer is revealed to be police detective Trotter. However, Trotter is an assumed name, and the man is actually not a police detective at all. It comes out that he is Georgie Corrigan, a 22-year-old whose younger siblings died due to terrible abuse when sent to live with a couple called the Lyons. Georgie murdered Mrs. Lyons in revenge for what happened, although she had already served jailed time.

While he is posing as Trotter, Georgie arrives at a snowed-in guest home called Monkswell Manor saying he is investigating the Lyons murder. While he is at Monkswell, the phone line is cut, and Mrs. Boyle is murdered. Mrs. Boyle was the woman who placed the Corrigans in their abusive foster home. Georgie also tries to strangle Mollie Ralston, one of the owners of the guest house, saying that as the Corrigans' former teacher, she should have interceded to prevent their deaths. However, he is interrupted during the strangling and apprehended.

The Mousetrap is a murder mystery in which the true culprit is the man you would least expect, in this case, a member of the police dispatched to help the Ralstons in the first place. However, like most things in this play, Sergeant Trotter is not at all what he seems, and in the final Act of the play, the character's insidious motives are revealed.

Trotter calls Mollie in the room after demanding to speak with everyone in private. It is revealed that Trotter is no police officer at all. His true identity is that of Georgie, the eldest Corrigan brother. It is revealed that he blames Mollie for the horrible fate of the Corrigan boys and intends to take his revenge on her.

Trotter (Georgie) is the murderer. This is openly revealed late in Act II. (See the section when he pulls a gun on Mollie.)

Greg