Mary was shown happily greeting her husband as he returned home from work, as she mentioned that she learned the gender of their unborn child (a boy) and was set on fixing their supper. However, Mary's joy is short-lived when Patrick tells her that he's leaving her and plans to marry another woman. Though she is heartbroken over the news, Mary's mind was solely on cooking, as she believed that Patrick would feel better once he has eaten.
Mary retrieves a leg of lamb from the freezer, and later pleads with Patrick not to leave her. Patrick responds by telling Mary to "try and stop him," and Mary does--by bludgeoning Patrick with the frozen leg of lamb, killing him instantly. After seeing that she has killed her husband, the evil Mary began to cover her tracks, first by placing the leg of lamb (the murder weapon) inside the oven and cooking it. She also sets up the scene to make it appear that Patrick was killed in a struggle, which included pretending to arrive and express shock over seeing her husband's body.
Mary called the police, who questioned her about the murder. After the interview, the detectives conclude that an intruder broke in and killed Patrick, though they could not find the murder weapon. Later on, Jack Noonan, the lead detective, opens the oven and notices the lamb inside, now well done, and it was at that moment that Mary offered the lamb to the policemen due to the fact that they were working past the dinner hour. The final scenes showed the detectives dining on the lamb, while Mary sat in the living room. The villainess heard the detectives discussing the location of the murder weapon, and gave a sinister laugh at Noonan's comment that the weapon was "probably right under their noses." Though it appeared that Mary got away with murder at episode's end, Hitchcock's closing narration revealed that Mary attempted to kill her second husband in the same fashion, but because the freezer wasn't plugged in, the lamb wasn't as hard, and she ended up arrested (off-screen) for the attempt and (assumingly) for Patrick's murder.