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?
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.
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 announces that he wants to become a full-time monster his parents start to worry.
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 tries to stop the post of his report while Kate and Nick try to prove who is brainier.
The monsters become obsessed with all things shiny - especially money, the shiniest thing there is. So, to get as much of it as possible, they set up a business in the basement.
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.
Norman turns into a human boy for just one hour and escapes from the basement - so Eddie and the others have to catch him before he barges in on Kate and Nick's dinner party upstairs.
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.
Eddie and the monsters keep reading Angela's diary, obsessed with her day-to-day life.
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 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 Kate wins a cookie competition, little does she know that some of Norman's fur went into the mix that won her the top prize. So when a film crew comes to film her making the famous cookies, Eddie and the monsters have to secretly get more fur into the dough.
The Carlsons are all set to go on a dream holiday to Mexico - until Norman eats their passports at the last minute. Eddie and the monsters soon persuade the family they don't need to go away to have fun.
Eddie pretends to be ill to avoid a maths test, promising the monsters they will have a great day hanging out together. But the monsters think they really have caught something - from Nick, who is at home, genuinely ill. So they make their own medicine which they test on Human Dad Thingy first.
When Eddie goes on a school trip, the monsters are lost without their leader. Fiend appoints himself king of the monsters, but Haggis and Norman each think they should be in charge. It is decided to have an election.
Eddie pretends to be ill to avoid a maths test, promising the monsters they'll have a great day hanging out together, but the monsters think they really have caught something from Nick, who is genuinely ill.
The monsters are fed up with the family never having time for them, so they steal all the clocks in the house. In response, Nick installs a huge and terrifying grandfather clock from now on, the house is going to run like clockwork.
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.
The monsters are left home alone while the family visit Kate's parents, they receive a surprise visit from a couple of burglars. The burglars get more than they bargained for as the monsters defend the Carlson house.
When the monsters are impressed by a few of Dad's old magic tricks, Fiend decides to become a master magician. Of course, he is useless, and manages to chain Angela and Norman together just as Angela has a date coming over.
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.
Nick and Kate go out for dinner, leaving a new babysitter in charge. At first it seems Mrs Travers just wants to spoil Eddie and Angela's friday night fun, but it turns out she is a monster catcher.