The Unstoppable Yellow Yeti

Leaving from a summery island, Osmo and his dad Nick are going to a town called Winterton to meet his cousin Rita. As they arrive, Mayor Chrome tours them while thinking that monsters aren't real. However, Osmo notices that his surf board is missing and tries finding it. He only finds half of the board with some big foot steps. Following them, he finds an area with someone in it and gives him the other half. Coming out of the area is a giant yellow yeti named Gustav, which Osmo gets freaked out by at first, but noticed it speaking. Gustav confirms that there is other monsters in the town but he isn't allowed per the rules. Rita takes a peak noticing that monsters exist, so she gets an old man to sound the alarm, scaring Gustav, though Mayor still doesn't believe that monsters are real. The people of town start singing which scares Gustav more, so Osmo makes earmuffs for him. Thanks to Osmo yelling at Gustav, an avalanche starts to fall down. As Osmo rides on Gustav, Rita notices that she will get run over by them.

Everyone in Winterton has a ski armlet except for Osmo, as per the rules of the town that everyone should have ski armlets, Osmo has to earn one or else he is banished. Gustav starts to teach Osmo on how to ski, though his skiing is different from human skiing. Gustav gets Rita to teach Osmo on how to ski, though it all doesn't work out as he keeps bumping into trees. As Mayor reports to Osmo about his exam, smashing into a tree counts as an aromatic failure. Now he has to pass by challenge, with the race course being one of the tallest mountains. Gustav distracts Mayor by eating an ice cream cone with sprinkles which violates the rules, and gets Osmo to finally pass the Mayor. With the Mayor winning, he violates one of the rules by swallowing the ice cream and pushes Osmo. Osmo wins the race and gets his armlet.

Going to Winterton peak, the gang, alongside Nick and Rita's parents are on a camping trip. Osmo learns about the founders of the town, Sir Wallace Wint and Emma Ertton. Rita goes to the mountain to find where Ertton camps at. Anders starts following the trio to see what they are finding. They did lose the map, but Gustav senses a smell of the fish fins and finds it, which gives Rita excitement, and found where Ertton was camping. Gustav wants to badly but Rita wants it intact for historical purposes. The gang found a snow fort with Anders and his friends. They steal the fish fins and the gang tries getting it back, only for them to lose it. Only happens however that Anders has the fish fins already. He opens the can and smells the stink, which would later get into the camp of the parents of Osmo and Rita.

As Mayor got himself a new rug, Gustav thinks that the rug was the skin of his grandpa, and cries about it. Osmo, Rita and Gustav decide to take the rug to the monster mountain graveyard with a traditional yeti funeral. However, after an owl steals the rug, Gustav catches it while falling on branches, them resulting in the rug to rip. A little of the rug still remains however. While barring the remains, turns out however that that Gustav's grandpa is still alive and comes to graveyard to spend time with his dead friends. It also turns out that the rug was actually manufactured. Gustav's grandpa now uses the rug remains as a wig.

While fishing after Gustav breaks the ice on the lake, Osmo catches a sea monster named Octavia, which jumpscares Osmo and Rita. The roaring however was monster for "hello". When Octavia is outside for too long, she drys so she gets back in the water and gives the gang lots of fish. Octavia later pops out of the sink, so the gang distracts Rita's mother. The gang and Octavia would later go outside for fun. Gustav sprays Octavia whenever she gets dry. Later the gang is back inside, where Octavia would eat the humans. Gustav warns the kids about it per sign language. Gustav pretends to cook the kids as "children chilly". As Octavia gets dried-up, Gustav reveals that this is actually a trap by him and Octavia escapes in the sink with Gustav blowing her.

Gustav flushes a legendary toilet, which causes everyone in town to want to sit on it.

Rita must win against Anders in a sauna dance battle.

After a glacier that was supposed to break apart at the beginning of spring stays intact, everyone suddely falls in love with each other, much to Gustav's disgust. He starts to do some tasks for the Northern Lights in order to make the glacier break.