What is the narrative in "Lamb to the Slaughter"?
I'm assuming you're asking what the narrative is about.
It is the tale of Mary Maloney and the perfect murder. It is also the story of Mary's transformation from a docile wife whose very existence is devoted to her husband, Patrick, to a woman who is now able to take charge of her life.
Mary learns that Patrick is leaving her. The reason is never clear. This is distressing because she is pregnant and has no real means of income, though Patrick states he will look after her.
Not knowing what else to do, she begins to make supper, which happens to be a leg of lamb. However, before she is cognizant of what she is doing, she bashes her husband over the head, killing him instantly.
Then she begins to plot how she can get away with the murder. She rehearses what she will say to their butcher and heads to the store. Appearing calm and collected, she gets the necessary things for supper. When she returns home, she calls the police, who all happen to be friends of her late husband, who was an officer himself.
She states that she found him dead when she came back from the butcher. The butcher is questions, but he states that she seemed quite normal to him. It is clear they thing she might have done it, but she acts so genuine, they quickly underestimate her and rule her out as a suspect.
As the story ends, Mary asks the detectives to stay for supper and feeds them the murder weapon.