Mrs. Stevenson is ill and stays in bed. She is trying to call her husband but she keeps getting a busy signal.
Once she gets through, she hears two men planning a murder plot. The murder was made by a "client".
She tries to tell the police the details, like how the victim is a woman who lives near a railroad and that they will strike at 11:15. The police do not believe her.
The police? Sorry, wrong number
Police station, what is the emergency
She then gets a message from her husband that he will stay late at work. She then looks around her and realizes he is the client and she is the victim!
As she tries to call the police again, before the time runs out, she hears someone enter the house downstairs.
When she is getting to the police, a crook kills her as the 11:15 train goes by the house (therefore no one heard her scream). Once the policeman answers, the crook simply says," Sorry, wrong number."