Molly examines her shelf of special things at the back of Mack's stall. She chooses the tub of blue slimy stuff and remembers the day she got it. Caitlin is in her new den, but she's in a bit of a grump and doesn't want to share it with anyone. When Euan, Ruby and Ayesha try to get in, she becomes an even bigger grump. Ruby has dropped a book about monsters and this gives Caitlin an idea - she's going to pretend her den has a monster to scare the others away! Meanwhile, Moira is trying to track down a different kind of monster - a biscuit snaffling one. She asks Bob to put a camera up in her office to find out who has been eating all her biscuits. Caitlin tells everyone about the monster. His name is Grump, and he oozes blue slimy stuff. Ayesha is very sceptical, so Caitlin borrows a tub of blue slimy stuff from Mack's stall and smears it all around Moira's office. She calls Ayesha in to see the slime as evidence of Grump's existence, but Ayesha remains suspicious. Moira and Bob look through the camera footage to find the elusive biscuit snaffler. As they do, they not only discover the culprit but also see footage of Caitlin smearing the blue slimy stuff everywhere. Caitlin has to come clean and apologise to Ayesha, Ruby and Euan.