As "Lamb to the Slaughter" begins, Mary Maloney, who is six months pregnant, is patiently waiting for her husband, Patrick, to return home from his job as a police detective. Once Patrick arrives, Mary makes him a drink and sits beside him in blissful contentment. However, something is off about Patrick's demeanor—he is drinking far more whiskey than usual and seems very aloof. Mary is in for a rude awakening when Patrick reveals that he is leaving her. This sends Mary into a state of dazed shock. Unable to do anything but her usual evening routine, she fetches a frozen leg of lamb from the cellar for their dinner. However, once she brings it back upstairs, her husband tells her not to bother making food for him since he is leaving. Upon hearing this, Mary snaps and swings the leg of lamb at him as hard as she can. She lands a fatal blow on the back of her husband's head, and he falls to the ground. The shock of his body crashing to the ground makes her realize that she has killed him. She quickly devises a plan since she knows her unborn child will have to live with the consequences of her actions. Moving methodically, Mary first puts the leg of lamb in the oven. She then prepares to go to the store and practices speaking in a calm and natural manner. While at the store, she engages in a cheerful conversation with the storekeeper, Sam, taking care to mention that she needs some vegetables for her husband's supper. Once she returns home, genuine grief overcomes her as she sees her husband's dead body. She then calls the police, and some of her husband's colleagues come over to the house. A forensic investigation begins in earnest. Mary tells the detectives a fabricated story, and her alibi is quickly established once the police officers cross-check her story with the storekeeper. Based on what they whisper to each other, Mary realizes that the police do not suspect her due to her previously chirpy and devoted demeanor at the store. Since the detectives now suspect foul play, they search high and low for the murder weapon, which they surmise must be a large, blunt object that the killer abandoned somewhere around the house. Mary cooperates with the police and says that she is unable to leave the house due to her shock and distress. She later offers the kind and hardworking detectives some whiskey and dinner, which they hesitantly accept because it is past their suppertime. As they eat the cooked leg of lamb—the crucial piece of evidence—in the kitchen, one of the detectives remarks that the missing murder weapon must be right under their noses. Mary hears this and giggles to herself as the story ends.