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 matching outfits. But once Mrs. Arnold leaves, Marilyn and Carolyn turn into twin terrors! They trick Mallory so that she can't tell them apart and they act like spoiled brats. Taking care of Marilyn and Carolyn is a baby-sitting nightmare. But Mallory is a responsible member of the Baby-sitters Club and she's not giving up!