Mary Maloney, a pregnant woman, expects her husband Patrick to return home from his job as a local police detective. When he returns, Mary notices that he is acting strangely and assumes that he was tired from work. Patrick finally reveals to Mary what is making him act strangely. It is not explicitly said, but it is insinuated that he is leaving her. Seemingly in a trance, Mary fetches a large leg of lamb from the deep-freezer in the cellar to cook for their dinner.
Patrick angrily tells Mary not to make him any dinner, as he is going out. She strikes Patrick in the back of the head with the frozen lamb leg, killing him. Mary realizes that she has killed Patrick and is willing to accept the penalty. However, she was scared for her baby and is not sure whether it would live. She prepares the leg of lamb and places it in the oven. After practicing a cheerful routine, she visits the grocer to come up with an alibi. Returning home, she enters the room with her dead husband lying on the floor and calls the police.
When the police arrive, they ask Mary questions and look at the scene. Considering Mary above suspicion because Patrick was a detective, the police conclude that Patrick was killed with a large blunt object, likely made of metal. After a fruitless search around the house and surrounding area, Mary offers the policemen the cooked leg, which they accept. When the lamb is nearly finished, the police discuss the murder weapon's possible location. One officer says, "Probably right under our very noses". The story ends with Mary overhearing their conversation in another room, and giggling, knowing that the last line is completely true.