Mallory knows she's a good baby-sitter. She's been looking after her seven younger brothers and sisters for years. So when Kristy offers her a steady job baby-sitting for the Arnold twins, Mal thinks it'll be easy money. The twins sure are cute in their little matching outfits. But once Mrs. Arnold leaves, Marilyn and Carolyn turn into twin terrors! They run around the house like spoiled brats, and Mallory can't even tell them apart. Taking care of Marilyn and Carolyn is a baby-sitting nightmare. But Mallory's a responsible member of the Baby-sitters Club -- and she's not giving up!