Mary Maloney, a devoted, obsessed and pregnant housewife, expects her husband Patrick to return home from his job as a local police detective. When he returns, Mary notices that he is acting weird and assumes that he was tired from work. Patrick reveals to Mary what is making him act strangely. Dahl does not reveal what is discussed, but it is presumed that Patrick tells Mary that he was going to leave her for another woman.
Apparently in a trance, Mary picks a large leg of lamb from the deep-freezer in the cellar to cook for dinner. Patrick angrily tells Mary not to make him any dinner, as he is going out. She hit Patrick in the back of the head with the frozen lamb leg, killing him.
Mary realizes that she has killed Patrick and has to create a story to tell the detectives. She prepares the leg of lamb that she has killed her husband with and places it in the oven to somewhat destroy the evidence. After practicing a cheerful routine, she visits the supermarket to establish an alibi. Upon returning, she enters the room with her dead husband lying on the floor and calls the police.
When the police (who were all friends of her husband) arrive, they ask Mary questions and look at the scene. Considering Mary above suspicion the police conclude that Patrick was killed with a large heavy object, likely made of metal. After searching around the house and surrounding area without getting any result, Mary is reminded the leg is cooking, and offers it to the policemen, which after doubting they accept. During the meal they discuss the murder weapon's possible location. One officer says it is "Probably right under our very noses". Mary overhears the last line and begins to giggle.