The best way to answer this question is to read the story. If you haven't already, I encourage you to do so. It isn't very long, and it's one of the most entertaining short stories out there. I have provided a link to the text of the story, and to some more detailed analyses and summaries of it. To read a summary of it without reading the story is to by seriously missing out. If you are having problems with summarizing and HAVE read the story, keep in mind that a summary is a brief overview of the main events that occur in the story itself.
In a nutshell, Mary Maloney is a cop's wife, and 6 months pregnant. She is happy in her marriage, and waiting for her husband to come home after work. He does, and informs her that he is leaving her. In a daze, she goes downstairs to get a leg of lamb out of the freezer to cook for dinner, goes upstairs, and without even thinking, bashes her hubby over the head with it, killing him instantly. Shocked, she realizes what she has done, and forms a plan. She puts the lamb in the oven, goes to the corner store to buy some food, making sure to talk to the clerk about how her husband is due home any time and she is making him dinner. When she gets home she calls the cops in a "panic" saying she came home from the store to find her husband dead. They come over, are confused, can't find a murder weapon. She feeds them the lamb for dinner, and they leave.
I hope that helps! Please, please go back and read the story also, it's a pretty cool one. Good luck!
The story opens with a description of Mary Maloney. She appears to be a contented housewife, waiting eagerly for her husband, Patrick, to come home from work. She is completely devoted to Patrick. So, it comes as no surprise that she is devastated when he returns home and informs her that he is leaving her. The reader does not know what Patrick's reasoning is. The reader is left with Mary's reaction of "puzzled horror." In shock, she goes to make dinner, returns with a frozen leg of lamb, and uses the lamb to bash her husband over the head, killing him.
At the beginning of the story, Mary appeared to very dependent upon her husband. It would not be an exaggeration to say she lived for him. But, after killing him, Mary shows great independence. It's as if a switch is pulled and she becomes another person. She immediately focuses on the problem and devises a strategy. She plays the part perfectly, gives herself an alibi, and brilliantly (albeit diabolically) persuades the police to eat the murder weapon. Indeed, it is the "perfect murder" as the previous post says. The beauty of this story is the ironic twist with Mary's character and the dark humor that concludes the story.
You can read an excellent summary of this story in the eNotes study guide. Briefly, the story is about the "perfect" murder. Patrick Maloney comes home from his work as a police officer and announces to his wife, Mary, that he is going to leave her. Mary, who is six months pregnant, cannot believe what she has heard, and she goes through the motions of cooking dinner. She gets a leg of lamb from the basement freezer, and when she sees Patrick standing with his back to her, without thinking she hits him over the head, instantly killing him. Now in a panic, she has to think quickly. She puts the lamb in the oven and hurries out to the grocery store to create an alibi for herself. When she returns home, she calls the police. She can honestly tell them that she found her husband dead when she returned from the store. The police search the house, but they find no clues. They even join Mary in a meal of leg of lamb.
FYI, this story was used as an episode on the old "Alfred Hitchcock Presents" series many years ago.