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!