Please provide a brief summary of the story "Lamb to the Slaughter."

The story begins with the pregnant Mary Maloney waiting
for her husband, a police officer, to arrive home. The way she looks after the house and
cares for her husband clearly demonstrates her devotion to him, and this is reinforced
by comments that show that Mary Maloney builds her life around her husband. However,
when he does finally arrive, she notices that something is wrong. He tells her that he
will be leaving her, presumably because of another woman. Shocked, Mary goes to the
freezer to get a leg of lamb to cook for dinner. Her husband says that he is going to go
out and Mary hits him on the head with the frozen leg of lamb, killing him
instantly.

Although she is not worried about her own life,
she is worried about the life of her unborn child, and so she carefully plans how to get
away with the murder. She puts the lamb in the oven and starts to cook it and then goes
to the grocery shop, planning her alibi. When she returns from the grocery store, having
kept everything "natural," she "finds" her husband's body lying on the floor and calls
the police in a shocked state, telling them that Patrick was murdered whilst she was
out. Once the police arrive, who all know her of course, they start looking for the
murder weapon, which to them always leads to the conviction of the murder. Mary asks
them to eat the meal she had cooked for Patrick for her and the story ends as she
listens to them eating the murder weapon that they are looking for, knowing she has
escaped being linked to the crime.