Big wakes up one morning and thinks: something is missing from my house. While he is looking for this something there's a knock at the door. And in walks Small. Big's first ever visitor; looking to have fun. Big, excited to have a guest, shows Small the garden swing that's hanging from the apple tree. But zooming through the air on a big swing isn't a lot of fun for a Small, especially when he ends up with a smooshed nose. What is fun, however, is a small model car that Big brings out of his tool shed. Fun, you see, comes in different sizes. And watching Small race around the room makes Big realize that's what is missing from the house. A Small - and a friend. A best friend. And Small, too, has found his best friend and a home. When Small knocks on Big's front door, Big's life will never be the same again!

Big decides to help Small by building him a teeny tiny itsy bitsy front door. Big is confused when Small tells him rather huffily that he likes the big door, that the big door is A-OK. Big sighs about not knowing what he's done wrong and looks through the small door with regret at apparently upsetting his friend. Uh oh. Big finds himself stuck in this narrow opening - and has to call out for help. Big realises that this is the first time he has ever asked Small for help - and that he should, of course, have asked Small if he wanted help with the front door before just going ahead. Small leaps to the occasion and, with judicious use of butter and pulleys, yanks Big free. Small admits that he knows Big only had good intentions with the door - and they decide to celebrate their friendship by going off to tickle the frogs. Big finds himself in a sticky situation when he looks through Small's new door!

Small sees a fish in the garden pond and immediately wants to put it in a bowl for a house pet. Big, who's busy building a boat, thinks that fishes are better off left where they are, in the water. Small, however, just knows that this fish will be the best pet ever and early one morning creeps down to the pond on a mission to bring him back to the house. Small finds the fish - but is suddenly dragged and yanked through the water by what is one ferocious aquatic animal! Small staggers back to the house, wondering if a worm would make a better pet. Small wants the fish in Big's pond for a pet - but the fish has other ideas!

Big thinks that Small is creating a mess all over the house and invents a machine which tidies away all of Small's debris. Small, shocked, tells Big that the pile of dirty socks Big has just sucked into his machine is actually a scientific experiment and that the old red jelly lid is a model of the planet Mars! Small decides to walk away and find some place else to live in - and discovers, through a mouse hole, a neat little room where he builds a fort from a sofa and bounces on the bed... 'What is going on in here!' cries Ruby, the pink mouse, whose home it is. She is not impressed with the fort. Big intervenes to calm her down and decides that the best thing to do is build Small a space of his own where he can be - well, where he can be Small! Big and Small introduce themselves properly to Ruby. Small drives through a hole in the wall and discovers a mysterious room.

When Big is building his latest invention, a Sunshine Catcher, he loses the doohickey that is integral to the machine's operation. He sinks into a deep funk and lay on his bed fiddling with the pieces of a puzzle. Unknown to Big, Small had put the doohickey into his collecting bag of bright, shining objects, totally unaware that it was the actual, important doohickey. Small endeavours to cheer up Big by trying hugs and pulling funny faces and doing some cool dancing... nothing works. Small then hits on the idea of making Big a cheer-up cake. He's sure that will work. When the cake is baked and iced Small thinks it needs a finishing touch: and he uses the shining doohickey from his collection. When Big sees the cake he sees the missing item and leaps happily from the bed. Result! Will Small's cool dancing cheer up Big?

Small wants the glorious apple that's hanging from the tree, even though Big tells him it's not yet ripe... But Small looks so sad that Big puts friendship above his knowledge of apples and picks the fruit. But 'Yuck! That's not an apple, that's a lemon!' is Small's reaction to the first bite. Big takes a bite himself to see if it really is that sour - and recoils with horror when a voice speaks to him from inside his mouth! He has swallowed something that lived in the apple! Small reaches in to free the trapped creature - who turns out to be a sassy but warm-hearted worm. She doesn't have a name but suggest they call her Twiba: The Worm In Big's Apple. And now Big and Small have a new friend out in the garden. Small asks Big to pick the juiciest apple on the tree - but the apple is home to a talkative green worm!

Big's number one favourite possession is his T-Rex: a beautiful, bashed up toy. Small, however, finds T-Rex terrifying with his pointy teeth and long tail. One morning T-Rex accidentally tumbles into a hole in the garden and Small sort-of-accidentally buries him... Big is so upset when he can't find T-Rex that all he can do is mope around the house and sing about how he misses his 'saur. Eventually, Small admits what happened. Big is shocked: how could Small do such a thing? But when Big finds out that Small is actually frightened of the toy, he understands and forgives. And Small admits that he knows he really shouldn't have buried his best friend's toy and won't do it again... Big's T-Rex toy goes missing. Big is upset but will Small tell the truth?

Twiba tells Small that a thunderstorm is coming - she can tell because the leaves in her tree are starting to shake. Small has never heard thunder or seen lightning and is really excited. Big, however, appears worried by the prospect of a storm and decides to sit it out in a closet under the stairs, taking a pie with him for comfort). When Small hears the first loud cracks of thunder he is startled not excited and runs into the house calling out for Big and eventually joining him under the stairs. Neither admits they are scared of the storm, however, they just say that they just prefer to remain indoors... But then they hear Ruby calling for help and have to venture out of the closet. It turns out that Ruby has only dropped some cheese and needs help carrying it back to her hole?

It's the day of the month when Big walks off to collect the mail - and so the day that Small can be the boss of the house. But before Big leaves he hands Small a long, long list of things Small Shouldn't Do. These start off sounding reasonable (don't dance on top of the sink) but quickly become extreme: no music and no games. No fun! complains Small. Left alone, Small decides to tear up the list and have fun instead (which includes dancing on top of the sink). However, it's no fun having fun on your own and Small very quickly misses Big and wants him to come home. When Big does arrive back at the house he sees a sad-looking Small and realises that his list was far too strict. Small should be having a good time in the house. Before going out for the day Big writes a long list of house rules that Small doesn't like at all!

Big and Small wake up to a creepy sound coming from downstairs. When they investigate it, they discover that it's just Ruby practicing her song. Ruby is having trouble, so Big and Small decide to try helping her. However, they both start to diverge from her idea, which angers her, as she feels like her song is perfectly fine.

Eventually, Ruby has enough, and snaps at the two. They didn't realize that they were angering her that much, so they both decide to help her sing it the way she wants it, not the way they want it.

It's a very hot day, and Big sees that his flowers aren't doing too well. He tries to use the hose on them, but Small has cut the hose in order to use it as a rope. Small doesn't want Big to fix the 'rope', so he tries to find something else that can bring water to the flowers.

Small eventually comes back with a mouthwash cap. Big tells Small that his plan isn't going to work, and goes to fix the hose himself. Small still wants to keep it, so he tries to convince Big to do a rain dance instead. Big is reluctant at first, but eventually decides to go along with it.

Small is talking about how much he wants a pet, when he discovers the perfect one - a 'dog' named Fang. Small instantly loves Fang, and begins training him in order to convince Big to let him stay. Eventually, Small wants to introduce Fang to Big, so he goes looking for him outside.

Big comes down to the living room, and sees a sock on the table. He takes it and puts it in the rest of the laundry, before putting it in the wash. Small then comes back inside, and sees Big. Small is excited to show Fang to Big, but there's a new problem: Fang is missing! Big is very scared of a dog named Fang being on the loose, but after seeing Small about to cry, he decides to help look for Fang..

Big and Small are outside at night, watching stars. Small is annoyed, as he thinks that stars aren't interesting at all, but after Big tells him that stars are actually big balls of fire, Small becomes more curious about them. He asks Big how many stars are in the sky, but Big isn't sure. After the two check a book about stars, they read that nobody knows for sure how many stars exist, so Big decides to be the first one to count them all.

Small heads back inside, and Big begins counting the stars, keeping track of his count on a clipboard. He quickly gets interrupted by Small, though, and loses track of the stars he's already seen. He tries counting again, but still gets interrupted by Small. Annoyed, but not wanting to give up yet, he starts counting yet again. This time, he gets very far, counting up to the 200s. Small is getting bothered by all of Big's counting, though, so he asks if Big is done yet.

Big loses track again, so this time, Small tries to help him.