When the story begins, Mary is eagerly awaiting the return of her husband. The narrator describes the scene of a cosy, comfortably family home and Mary is completely enamored with her husband, Patrick.
She loved to luxuriate in the presence of this man, and to feel - almost as a sunbather feels the sun - that warm male glow that came out of him to her when they were alone together.
So, the first minor surprise is when Patrick explains to Mary that he's leaving her/divorcing her. This is never blatantly expressed but it is clearly implied. The next surprise is that Mary, who seems subservient and dedicated to this man, does not resort to crying or pleading. Rather, she uses the leg of lamb to strike and kill Patrick (hence the title). But the real surprise is when the seemingly meek and dependent Mary devises an elaborate scheme to avoid going to jail. She uses her intelligence and cunning to get rid of the evidence by feeding it to the police. Mary's violent outburst after being rejected is surprising enough. But the lengths she goes to (and the imaginative strategy of eliminating the evidence against her) is even more surprising. At the end of the story, as the policemen are ironically discussing the murder weapon while eating it, Mary lets out a giggle in the other room; again, another surprise that this is not the same Mary as described at the beginning of the story.