King

The Wright family moves to a new house and Russell discovers the passage to Under under his bed. He befriends Loopy and Vernon and together with Russell's dog Gus, they challenge Bob Wire for the crown of Under and Russell becomes Under's new King.

Bob sends a fake SOS to Russell from ex-Princess Populah so he can lure him into a trap and steal back the crown. Meanwhile, Loopy hatches what he believes is an Urklet egg, only to find it was a vicious Rebbuloc, which terrorizes the throne room before escaping to Up and encountering Cliff, who was trying to sneak down to Under.

While trespassing in Near Under, Loopy is mistaken as a pet by Auntie First and she locks him away. When Russell goes to help, Gus is also taken and Populah has to help the King rescue his friends. After leaving for Near Under, Vernon starts work on Under's new currency: greenbacks, which Bob quickly began hoarding to himself.

By Russell's request, Vernon builds a car. However, Bob steals it to bring to Auntie First as their new secret weapon for invading Under, leaving Russell and his friends stranded in the desert.

Russell orders the populace of Under to stop blaming the Pubrick for all their problems, not knowing that this will allow the creature to turn into a fearsome beast. Meanwhile, Bob commandeers Vernon's ripulator to create a fault around Under, ransoming the Crown in exchange for the key that will set things right and prevent Night Slythers from emerging at sundown and destroying everything.

When a stray dog keeps him up all night, Russell goes down to Under and finds everybody sending their garbage up to the moon, and Gus and the crown are sucked away as well. When he goes to retrieve them, Russell learns from the Hermit who lives on the moon that one more load of garbage will send it crashing into Under next year.

Mysterious pollen flows from Under up into Russell's room, knocking out Cliff and sending Russell down to investigate with Gus. When they get rid of the bush responsible, Loopy discovers that a mass of unknown monsters are bearing down on the town.

Bob tries to trick Auntie into invading Under, eventually carving a Floating Mountain into the shape of Russell's head that motivates Auntie to build a giant wall. This forces the Gorbal Birds to fly to Under instead of to their normal roost in Near Under. This upsets Russell since the birds start pooping on his newly constructed Roller Coaster (which he built since he's too short to ride one in Up).

While doing some plumbing, Russell and his friends are sucked down to Under Under where the crown is eaten by a Slyther. They befriend her owner Hugh the Yu-Yu, who is living under a giant stalactite that is poised to fall at any moment. Since his Fuzzy Dice told him to stay, Hugh, despite Russell's insistence refuses to leave.

Russell decides to practice for his Gopher Scout camping merit badge in Under. Vernon dislikes living off the land and sets up a big fancy forest camp for all the people of Under. Meanwhile, Bob steals Russell's scout handbook and tries to use its advice to steal the crown.

Vernon and Loopy discover that Russell is upset about not making his school football team, and make him part of Under's Joustabout team (like simplified Quidditch on flying snowboards), rigging all the games so Russell would win. But when he discovers this, Bob makes a killing off of gambling bets, and soon moves onto betting the crown of Under on a one-on-one match between him and Russell.

Russell starts masquerading as his comic book hero 'The Coyote' but when no crime presents itself, Vernon starts making unreasonable laws that people can't possibly abide by. Loopy also becomes involved with The Coyote and takes on the role of the villain, 'The Grim'.

After accidentally turning Under into pumpernickel, Loopy runs away to be a pirate. Russell, Populah, and Gus go after him and are captured by Loopy's new crewmates. Meanwhile, Vernon tries to stop Bob as he sends rock monkeys to eat Under.

While trying to get down to under, Cliff is bitten by a Frobish, catching Monsteritis and thus turning into a monster himself. Russell goes to the Uncertain City to get the slime of a fairly large Dongo Worm, the only cure for Cliff. Bob and Auntie also want the worm for their own uses, and Gus tries to organize with Captain Darling and the Frags so they can capture the worm together and escape the city before it blinks out of existence.

Gus digs up a dinosaur bone in Up but when he takes it down to Under, Vernon's latest gizmo reanimates it. Gus feels Russell loves the dinosaur more than Russell does Gus, so Loopy takes him to find the Everything Log so he can wish it away. But Auntie had already gone missing looking for it, and Bob decides to try to find it himself too.

In order to make ice cream, Vernon and Loopy set up a factory with a blizzard inside it, causing the outside world to turn into a desert. When Russell decides to get rid of the ice cream in favor of the climate, Bob starts a coup. Meanwhile, Near Under is beset by the petrifying Hairy Eye-Ball, so Auntie and Russell make a deal to for each to solve the problem of the other.

Bob uses a monstrous Nossix to take over Under and steal the crown. Russell escapes and heads to the cruel Tower of Derision with Populah to read Great Great Uncle Zeb's advice on beating the Nossix. The Frags, having been outed by the monster, teach Vernon and Loopy how to indirectly disobey Bob's undesirable orders to stop Russell.

Faced with a riddle he can't solve, Vernon breaks down and Russell takes him to the Clockmaker to be fixed. Unfortunately, the Clockmaker won't cooperate, needing Vernon to keep time correctly in his town. Meanwhile, Loopy helps Auntie first stop Bob from demolishing Near Under as part of his "Wire Man" show.

Gus chases a cat to Under and she takes Bob hostage, using him to fly her away from Gus to Near Under, the dog and the Frags (who don't understand the dog-cat feud anyway, believing peace was better) in hot pursuit. However, Auntie First is waging a war with her Cousin Tess, the leader of Under Sea because they once wore the same dress to their school prom. Russell tries to stop the war since all the sea creatures are being forced to flee to Under due to it. However, once Russell saw that the dresses were not the same, the evidence was destroyed.

Embarrassed by the prospect of trading party supplies to Hugh the Yu-Yu for her weight in Slyther Cheese, Auntie (with help from Bob) kidnaps Loopy to make her his Lighter Than Air pastries. Without the trade, though, Hugh is faced with a giant rat attack, and Russell must step in to save the economy.

The mouse-like bandits start stealing things in both Under and Up, leading the Wrights to believe the house is infested by rodents. They then call an exterminator and Russell fears the passage to Under will be discovered, he is also blamed by Lou and his mother for the thefts.

In order to go river rafting, Russell has Vernon create a Veggi-Double clone to attend the G-4 meeting in his stead. But when a Giga Gut attacks Near Under, Auntie First, Cousin Tess and Hugh the Yu-Yu find "Russell" to be no help at all in finding a solution, and the situation worsens when they are informed by Bob Wire that they were dealing with a clone.

When it does its annual flyby to Under, the Bluebird of Happenstance poops on Loopy bestowing him with bad luck and takes off with Russell's crown, leaving a blue feather behind. Loopy locks himself inside a rollable glass ball to stop getting bad luck while Russell tracks down the bluebird. Meanwhile, Auntie causes her subjects to run away again to Under over an unfair "Light Tax" and has to rely on Bob to bring them back when he gets the crown from the bluebird.

When he loots the Uncertain City, Bob comes across three ancient urns. Auntie confiscates one with a freeze-dried Giant Nose (an extinct monster) which comes back to life and starts attacking the towns. Loopy buys an urn filled with ancient air from Bob and releases it in the castle, sending it and everyone inside back to prehistoric times.

Russell infects all of Under with his cold, turning them all into uppity superheroes who kick Russell and Gus out of the town. Bob and Vernon, being inorganic, were immune. They go to Near Under and Captain Darling tells them a plant native to the Equatorial Glaciers will cure the infected Underites, but Bob tells their plan to the Underites in an attempt to get into their good books. Vernon tries to find the cure for Russell's cold in Up.

Faced with a daunting school recital, Russell has a recurring nightmare about performing the accordion in front of a hoard of monsters. In Under, the Dream Gulper arrives and materializes the dreams of its sleepers. Bob catches the Dream Gulper for his dream of being a giant and ruling all the local kingdoms to come true. Russell has to confront his fears to reach the Dream Gulper and stop Bob's dream from becoming reality forever at sunset.