Detective Maloney tells his wife that he's leaving her. She is a shock and goes downstairs to get the frozen leg of lamb for dinner.
In her shock, when she comes upstairs she hits him on the back of his head with the lamb, killing him. She knows that the punishment for killing him will be high so she puts the lamb in the oven and goes to the store and buys potatoes, peas and cheesecake
When she comes back and sees her husband's body she starts crying and calls the police department. She tells them to come because her husband is dead.
“Probably right under our very noses. What you think, Jack?
When the police arrive and see the body they immediately rule out Mary as the culprit because her alibi made perfect sense. In addition to that, she was pregnant so why would she kill her husband?
The officers determined that Detective Maloney must've been killed with a metal rod. Mary, noticing the lamb in the oven was done, offered the officers some. When they refused she pleaded them, and they eventually gave in.
The officers happily enjoy the lamb exchanging ideas of where the murder weapon could be.