It's Eddie's birthday and he can't wait for a party with his three best friends in the all the world - Haggis, Fiend and Norman. But when the monsters argue over what present to give Eddie, it turns into a full-scale row that threatens all the party plans, and even their friendship with Eddie.

When Eddie tells the monsters about swapping teeny pets - free toys that come in cereal packets - Fiend becomes addicted, determined to build the biggest collection of teeny pets ever. Meanwhile, it gives Kate and Nick the idea to swap roles - leading to even more chaos in the household.