The story opens at the Maloney residence as Mary Maloney—six months pregnant and glowing—waits for her husband, Patrick, to return home from his job as a police officer. She has carefully prepared for his arrival; two glasses stand at the ready, and fresh ice cubes fill the ice bucket, as she eagerly anticipates the sounds that herald his return. It is a Thursday night, the night the couple tends to go out to eat. Like the domestic scene Mary arranges for Patrick’s return, this Thursday night ritual is habitual and represents the couple’s penchant for routine and consistency.

As gravel crunches outside and Patrick’s key turns in the lock, Mary rises to greet her husband. She returns his coat to the closet, pours his drink, then joins him in the living room. Seated in their respective chairs, Mary returns to sewing and waits for Patrick to finish his drink. As she does, Mary considers her husband, detailing to the reader the depth and passion of the love she feels for him This emotional insight is juxtaposed with Patrick’s limited dialogue; he is brusque and snappish, with seemingly little regard for Mary’s efforts. In response to her concern for his health, he does the unexpected, downing his drink in one go, then immediately pouring another, stronger than the first.

Alarmed by this disruption to the pattern of their peaceful domesticity, Mary offers to pour Patrick another glass, fetch his slippers, and cook him dinner. Her attempts to put Patrick at ease seem to frustrate him further, and he tells her to sit down and listen. He then reveals that he plans to leave her and their unborn child. While he promises to ensure they are adequately cared for, his speech is cold and uncompromising, ending with the callous request that Mary put up little fuss for the sake of his job.

Dazed by this revelation, Mary reflexively returns to the familiar habits of everyday life and attempts to cope by making dinner. From the cellar freezer, she retrieves a leg of lamb. On the way to the kitchen, she passes Patrick, who rudely tells her not to make him dinner, for he is leaving. Impulsively, Mary arcs the leg of lamb high in the air and brings it crashing down onto the back of Patrick’s head, killing him. He wavers briefly before thudding to the floor. Quickly, the shock begins to fade, and Mary realizes what she has done. Her mind clears, and she begins to consider the ramifications of her actions. As a police officer’s wife, she is well aware of the consequences of murder. Concerned with the well-being of her unborn child, she realizes that she must disguise her guilt and begins to formulate an alibi.

She places the leg of lamb in the oven, then begins to put herself back together, fixing her hair and makeup, and practicing her behavior in the mirror. After composing herself, she leaves for the grocer’s under the guise of purchasing vegetables for dinner at home. There, she chats nonchalantly with the grocer, Sam, and maintains a convincing veil of casual ignorance. Her performance is so successful that, as she heads home, she begins to convince herself of her own innocence. Mary imagines the scene she will return home to, envisioning her husband, tired from a taxing day at work, waiting patiently in his chair for her to finish dinner. Returning home, she is, of course, actually greeted by Patrick’s dead body, contorted and growing cold. Rediscovering this grotesque scene sends Mary into convulsive sorrow, for after all, this was the man she once loved dearly.

Tear-stricken, she calls the police. Two officers she knows from Patrick’s precinct, Sergeant Jack Noonan and his partner, O’Malley, arrive at the scene. She relays her story while the two men investigate the body. They quickly notice signs of foul play, and O’Malley calls for backup. More officers arrive at the Maloney residence, including a police photographer, a doctor, and two detectives, who begin to question Mary. When she mentions her trip to the grocer’s, one of the detectives leaves to interview Sam. As he discusses his findings, Mary overhears snatches of the conversation, just enough to learn that she played her part well enough to convince Sam. Her alibi is air-tight.

Sergeant Noonan suggests that it might be better if she retired from the stress of the situation. He opens his home to her and offers a number of alternatives so that she might leave the distressing scene. Mary pretends to be too overwhelmed to move and asks to stay in the living room until she feels better. In reality, she must remain at the scene to complete the remainder of her plan. As she sits in the living room, the officers search the house. Sergeant Noonan checks in on Mary frequently. He tells her that they are searching for the murder weapon and that, due to the nature of Patrick’s injury, it must be unwieldy and difficult to hide. Despite this claim, the officers’ search stagnates, and they begin to grow weary. Hours of fruitless labor have yielded few results, and they are frustrated. Mary offers Noonan a drink, which he uncomfortably accepts. She plies her position as a pregnant, newly widowed housewife to convince the other officers to join them.

Noonan informs Mary that the oven is still on. She asks that he turn it off and that the officers eat the dinner she has so diligently prepared. She offers them the lamb, claiming that they would be doing her a favor by eating it. As the four men agree and adjourn to the kitchen, they unknowingly begin to consume the very evidence they sought. As the officers eat, they discuss their inability to locate the murder weapon. Mary listens as their voices drift into the living room; over belches and the sound of chewing, she hears an officer suggest that the murder weapon is “probably right under our very noses.” She begins to giggle quietly, realizing that her alibi was successful. The evidence has been eaten by those searching for it, and she has assured the safety of her unborn child.