Mary’s behavior at the grocery store contradicts what happened earlier in the story because she is in a completely different frame of mind.

Earlier in the story, Mary Maloney, who was in her sixth month of pregnancy, had just found out that her husband, Patrick, has decided to leave her. While she initially carries on with preparing to make supper, as if in some kind of a daze, fury soon overtakes her, and, as the old saying goes, “hell hath no fury like a woman scorned.” She is justifiably enraged that her husband has decided to leave her when she is due to have their child in just a few months.

What happens next, however, is not so justified. Instead of cooking the frozen leg of lamb that she has taken out of the refrigerator to prepare for dinner, she winds up using it as a murder weapon when she hits her would-be ex-husband over the head with it. While this took place, Mary was definitely not thinking clearly.

After she realizes that she has killed Patrick, Mary realizes she needs to act with the best interests of her unborn child at heart. She is now thinking clearly, and that is where the contradiction with her earlier actions lies. If she will be put to death, her child will die with her. She therefore puts together an elaborate plan for an alibi, in which she goes out to the grocery store and pretends it just an ordinary evening during which she is picking up supplies to make her husband’s dinner. She then goes home and calls the police as though she had just discovered Patrick’s body.

After the pregnant housewife Mary finds out that her husband is leaving her, she enters into a kind of a trance. She goes down to freezer and retrieves a leg of lamb. The text tells us that

She did everything without thinking.

After she whacks her husband on the head with the frozen leg of lamb and kills him, she snaps out of her stunned trance:

It was extraordinary, now, how clear her mind became all of a sudden. She began thinking very fast.

Unlike her unthinking behavior when she was first stunned by her husband's unwelcome news, Mary now starts rapidly planning and, more to the point, acting. She goes to the grocery store precisely so she can establish an alibi and to pretend the murder took place while she was out. As she is getting ready to go to the store, she rehearses her lines to the grocer just as if she is an actress preparing to go on stage. As she is coming home, she mentally runs through the "scene" ahead in her mind and determines what kind of emotions she is supposed to exhibit:

if she found anything unusual or terrible when she got home, then it would be a shock and she would have to react with grief and horror.

She tells herself to behave naturally, but she is scripting the whole evening to be an act. She pretends her husband is still alive as she comes through the door:

"Patrick!" she called. "How are you, darling?"

She decides it is easy to pretend. Her old "love" for her husband comes flooding back so that when she starts to cry, it seems to her that she is hardly acting at all. But she clearly is playing a part.

Mary has just killed her husband in a fit of rage, after he had obviously told her he was leaving her and wanted a divorce. She does not want to go to prison, especially since she is expecting a baby and knows the baby would be taken away from her. She has to establish an alibi. She goes to the grocery store in the hope of making it look as if some intruder had been waiting for an opportunity to sneak inside and murder her husband. Since he has been a policeman for many years, he could have made many enemies by arresting them and having them sent to prison. Mary needs time. She wants that leg of lamb to be thoroughly cooked so that it won't be recognized as having been used as a blunt instrument. This is a perfect-crime story in which the perpetrator escapes punishment because the murder weapon is disposed of in a unique way.

Mary knows the grocer well, since she shops there all the time. She engages Sam in conversation and explains why she is doing this last-minute shopping.

"I want some potatoes, please, Sam. Yes, and perhaps a can of beans, too. Patrick's decided he's tired and he doesn't want to eat out tonight," she told him. "We usually go out on Thursdays, you know, and now I don't have any vegetables in the house."

This type of personal relationship between a small shopkeeper and a customer used to be far more common in cities than in today's supermarkets. Sam will make an excellent witness, if necessary, to establish that Mary was grocery-shopping at the time her husband was killed. Meanwhile that leg of lamb must be thawing fairly quickly in the oven. The author establishes that Patrick always gets home at five o'clock and that it is nearly nine o'clock when Mary suggests that the searchers have a drink and must be close to nine-thirty when she asks them to eat the leg of lamb. The lamb has been cooking for about four hours, which seems just long enough for a frozen leg of lamb to get fully cooked.

Time is of the essence here. The author has to account for those four hours so that he can deliver his comical ending in which the policemen are gobbling up the evidence they have been seeking ever since they arrived. While they are eating every last bit of the murder weapon, one of them says:

"Personally, I think the weapon is somewhere near the house."

And another officer replies:

"It's probably right under our noses. What do you think, Jack?"

In "Lamb to the Slaughter," how does Mary's behavior in the grocery shop contradict what has happened earlier in the story? Why is her visit to the grocery shop an important piece of the story?

Before Mary heads to the grocery shop, she kills her husband by hitting him in the head with a frozen leg of lamb. She then practices her "speech" to the grocer and proceeds to create a ruse that will exonerate her from any murder charges.

Once Mary kills her husband, she goes upstairs and rehearses what she will say to Sam, the grocer. It's not clear what she's planning as she contemplates her possible jail time. However, when she gets to the grocer, we see the behavior of someone who has faced no trauma or tragedy. Mary acts as if nothing ever happened. She orders some food, chats with Sam, and takes the food back to the house.

As she's walking back, the narrator reinforces her new storyline, telling the reader, "she must cook it good, and make it as tasty as possible because the poor man was tired." She is creating a new narrative that completely contradicts the fact that she just killed her husband. Her husband is not tired; he's dead! She is rewriting herself out of the narrative.

When she returns to the house, she "discovers" her husband on the floor, presumably dead, and reacts as such. She calls the police and tells them her new story of grabbing some extra food for dinner and returning home to find him injured on the floor. Now, back to the grocery shop.

Mary's trip to the grocery shop is important for a few reasons. First, it gives her a possible alibi in that someone now knows where she was during the time of the murder. Sam can attest to the fact that Mary was her "normal" self and picked up some of her husband's favorite foods. The trip also helps her push the food onto her guests, which in turn destroys the murder weapon and any evidence of her guilt. She has this large meal prepared that is reminding her of her newly deceased husband, and she guilts the cops into eating all the evidence. The trip to the grocery shop, while it contradicts the reality of her behaviors, becomes the linchpin in her manipulative plan.