The Carlsons move into their new dream home, but little do they realise there are monsters living in the basement. How long can Eddie hide Fiend, Haggis and Norman from the rest of the family?
Eddie tries to stop the post of his report while Kate and Nick try to prove who is brainier.
Haggis is depressed and no one knows why. A therapist says he needs a father figure, so Nick reluctantly steps into the role. But when Nick starts to enjoy his time with his new monster son, Eddie and the other monsters start to get jealous.
Eddie is convinced that his parents are splitting up and asks the monsters to help him stop his dad running away. But the monsters are too busy reviving their singing group, The Fabulous Monsterettes, convinced that Angela wants them as her backing group.
When Eddie teaches the monsters about lying, little does he realise what he has unleashed. What starts out as a tiny fib soon escalates into a series of huge whoppers, and Eddie has to play along with all of them for fear of being found out.
When Eddie teaches Haggis to be more assertive, things soon get out of hand as the furry giant starts to throw his weight around. Meanwhile Fiend is reduced to a gibbering wreck, now that Haggis has taken his role as the bossy monster.
When Nick messes up his and Kate's anniversary, Fiend, Haggis and Norman kidnap him.
When Angela's new-found friends bail out of her first ever sleepover, the monsters step in for some fun.
Eddie and the monsters keep reading Angela's diary, obsessed with her day-to-day life.
When the monsters scare off a cleaner, Kate reaches breaking point; either they clean up their own mess or get out. Eddie is desperate to tidy the house before she gets back but the monsters have other ideas.
When Angela is asked to babysit, the monsters ask Nick how babies are made. As usual, they get the wrong end of the stick and Haggis ends up thinking he's pregnant. But will Eddie and Angela persuade him that the baby's not his before the real mother comes back?
Eddie sneakily floods the basement so that Fiend, Haggis and Norman are allowed to live upstairs with the family. But his plan backfires when hanging round with the human's too long turns the monsters into very un-monsterly, civilised grown-ups.
When Eddie announces that he wants to become a full-time monster his parents start to worry.