She sat and waited for her husband until he came home from work. She wasn't making dinner because it was thursday and they go out for dinner on thursdays. When her husband finally comes home she notices something off.
When he comes home she makes him a drink and he downs it in one gulp. After a little time passes she goes to make him a leg of lamb for dinner. Instead of preparing it for dinner she hits him on the head with the meat. Then cooking after.
Mary goes to the super market to create an alibi. First she makes sure she sounds normal or as if she didn't just kill her husband. She asks the cashier for vegetables to go with her lamb she has cooking.
Mary called the police saying that someone killed her husband. One of the cops tell her that she left the leg of lamb in the oven still cooking. Mary tells the officers that it would make her feel better if they ate it.
The officers eat the lamb not knowing and not even thinking that it was murder weapon. The police talk about what he would've been killed with. While they all talk and eat the lamb, Mary laughs because she got away with murder.