Ever played fetch with a four-eyed Grerk or made pancakes with a giant orange squurm? Nelly isn't scared of monsters. In fact she babysits for them. Every night, Nelly the monster sitter looks after a new friend, but it's never easy...

Nelly can't believe her luck when she finds out that the Water Greeps live in an underwater penthouse. The only problem is Water Greeps are mischievous monsters, and Nelly is going to have to get a bit wet!