Describe the mood. How does it shift throughout the story?
The initial mood is set by Mary herself. Her home is "warm" and inviting as she waits for her husband, Patrick, to return from work. There are two glasses waiting for the two of them. She waits with a little anxious anticipation but she is "curiously peaceful." She is very comfortable with the stage and state of life that she's in. She is pregnant and her eyes have a "new calm look." The opening mood is intimate, warm, and peaceful. It indicates a happy family atmosphere.
Patrick does something "unusual" when he consumes his first drink in one gulp. Then Patrick repeatedly refuses Mary's offer to get him something to eat. He tells her to sit down and at this point, she becomes frightened. So, the mood shifts from peaceful to tense at this point.
Patrick reveals that he is leaving her. Dahl doesn't provide any concrete details about this. This adds to the suspense of the moment. With that suspense, the tension increases. The tensions reaches a climactic point when Mary hits him over the head with the frozen lamb. It is a violent and morbid image. "She stepped back, waiting, and the strange thing was that he remained standing there for at least four or five seconds. Then he crashed onto the carpet."
After this, Mary begins thinking about how to get away with the murder. The mood remains tense as she goes about putting her plan into action. When the police and detectives arrive, Mary remains calm or pretends to be calm. However, watching or reading the scene, one is probably tense and anxious about Mary's fate.
In the end, Mary has gotten away with it. The concluding mood is one of a dark, morbid resolution. A reader might even interpret it as darkly comical.