I would argue that there are two distinct plot twists in this great short story by Dahl.

The first is Mary Maloney's response to her husband's presumed affair. (The text never directly states that this is his news, but textual evidence supports this interpretation.) While she has been presented as a doting, encouraging, and loving wife, it turns out that even the best of wives can be pushed too far. After giving his wife the news and then telling her that "there shouldn't be any problem... It wouldn't be very good for [his] job," Patrick tries to dismiss his wife. Mary Maloney is so stunned that she stumbles around trying to continue the routine of their lives. Instinctively, she begins preparing dinner, just as she does for her husband on any other night. When he condescendingly instructs her to stop, first with an "I've already told you" and then by noting that he's going "out," likely to meet the other woman, there is a plot twist.

Mary Maloney takes the large, frozen leg of lamb and "without any pause" raises it high and then forcefully strikes Patrick in the back of the head, killing him instantly.

Patrick didn't see this coming, and he surely didn't think his wife capable of murder. Mary's calm reaction and methodical construction of an alibi are completely unexpected based on Mary's early characterization in the story.

A second plot twist comes near the end. Mary has thrown the leg of lamb into the oven, and the detectives are in her home trying to find the murder weapon. Telling the detectives that her husband would "never forgive her" if she failed to offer them something to eat, Mary serves up dinner, which brings the final plot twist.

The detectives eat the murder weapon.

The evidence against Mary Maloney is now destroyed by the detectives themselves.