The Wright family moves to a new house and Russel discovers the passage to Under under his bed. He befriends Loopy and Vernon and together with Russel's dog Gus, they challenge Bob Wire for the crown of Under and Russel becomes Under's new King.
Bob sends a fake SOS to Russel 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 actually 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 Russel 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 Russel's request, Vernon builds a car. Bob however steals it to bring to Auntie First as their new secret weapon for invading Under, leaving Russel and his friends stranded in the desert.
Russel 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 Russel up all night, he goes down to Under and finds everyone sending their garbage up to the moon and Gus with the crown are sucked away as well. When Russel goes to retrieve them, he learns from the Hermet 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 Russel's room, knocking out Cliff and sending Russel 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 Russel's head that motivating 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 Russel 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, Russel 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. Despite Russel's insistence, Hugh refuses to leave since his Fuzzy Dice told him to stay.
Russel 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 Russel's scout handbook and tries to use its advice to steal the crown.
Russell learns to play Under's national game for the first time: Joustabout. But will his limited experience be enough when he faces Bob Wire for the crown title?
Russell dress up as his favourite comic book superhero to fight crime in Under.
After accidentally turning Under into pumpernickel, Loopy runs away to be a pirate. Russel, Populah and Gus go after him and are captured by Loopy's new crew mates. Meanwhile, Vernon tries to stop Bob as he sends rock monkeys to eat Under.
A Frobish monster bites Cliff when he goes snooping in Russell's room, and Russell feels it is his responsibility to find the antidote before he becomes a monster permanently.
Gus searches for the Everything Log, independently, in order to make a wish.
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 Russel 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 Russel make a deal to solve each other's problems.
Bob uses a monstrous Nossix to take over Under and steal the crown. Russel 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 Russel.
Faced with a riddle he can't solve, Vernon breaks down and Russel takes him to the Clockmaker to be fixed, only 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. Russel tries to stop the war since all the sea creatures are being forced to flee to Under due to it. However, once Russel saw that the dresses weren't actually 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 (and Bob) kidnap Loopy to make her his Lighter Than Air pastries. Unfortunately, without the trade, Hugh is faced with a giant rat attack, and Russel 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 Russel 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 and Bob find "Russell" to be no help at all in finding a solution, until they are informed by Bob Wire that they were actually dealing with a clone.
When the Bluebird of Happenstance does its annual flyby to Under, it poops on Loopy bestowing him with bad luck and take off with Russel's crown, leaving a blue feather behind. Loopy locks himself inside a rollable glass ball to stop getting bad luck while Russel 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 Bob loots the Uncertain City, he comes across three ancient urns. Auntie confiscated 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.
Russel infects all of Under with his cold, turning them all into uppity superheros who kick Russel 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 Russel's cold in Up.
Faced with a daunting school recital, Russel has a recurring nightmare about performing the accordian in front 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 giant and ruling all the local kingdoms to come true. Russel has to confront his fears to reach the Dream Gulper and stopping Bob's dream from becoming reality forever at sunset.
When a month-long solar eclipse takes place in Under, it becomes overrun by Brain Suckers, turning all their victims into mindless drones. They can be 'resmartulated' with Vernon's solar powered Resmartulator, but Bob tricks Vernon into smartulating him without being brain-sucked first, making him a super genius (at least by Bob's standards) and able to successfully take over Under. To stop Bob, Russel also has himself super-smarted, but he becomes conceited and uncaring as a result.
Sick of being a hated despot, Auntie goes to the Kiosk of Random Opportunity to get a new job. Auntie becomes a "great big sacrifice" to the Great Big Gehh, a monster terrorizing the Valley of the Outies once defeated by ex-King Cliff. Loopy also gets tired of his job and heads to the Kiosk. When Russel and friends went to find Loopy, they find a light signal of Cliff coming from the Valley of the Outies.
Bob kidnaps the Truth Teller and has the Frags masquerade as her to make him King and to make Russel and Gus disappear. Unfortunately, he chose to act just as the Lurber rose from the ground, intent on eating its only prey: Kings.
The Big Cloud Mall kidnaps Auntie First, the Flatulent Swamp and Under after its Belch-Bean Cookoff in order to keep its engine (which runs on bad smells) running. Russell and friends head to the Mall to save Under but are slowed down by security. Fortunately Bob, seeing them on the Mall, realizes Under is going to be saved either way, and decides to do it himself and earn their undying gratitude.
Populah is sent to Miss Atilla's School for Young Despots by Auntie and Tess. With Populah not having any despot edge, she can never graduate. So, Russel, Loopy and Gus pretend to be girls and sneak into the school to help Populah win the despot derby for Populah for instant graduation. Bob also cross dresses to get in as a teacher in order to steal Russel's crown, which had been taken from him when he enrolled.
Loopy gives Russel an attachable tail as a King Appreciation Day gift, and Russel uses it to perform acrobatics at the travelling circus (ironically, the one Bob Wire was also working for). Unfortunately, when the circus is summoned to Emperor Lugobrius, they are faced with either making him happy with a good show or get shaved clean and locked away forever (like all the other entertainers who attempted to please him and failed).
Auntie First and Loopy are both swindled into getting the Ultimate Defensive Beasts: Krenits, thus both of their populaces had to remain perfectly still or be eaten by the monsters. Russel goes to get the Ultimate Offensive Beast, the Ippy Clippy, to get rid of them, but Bob beats them to the pumch and they are locked in the Wall Eye (where the Ippy Clippy had been stored by Great Great Uncle Zeb) with a flotilla of monster guppies.
After being weighed down by too much garbage the year before, the moon is about to crash into Under. Russel plans to stop it by becoming giant and smacking it away with a tennis racquet, but Bob steals the Enlargulator to make a new town for the Underites when the moon crashes, making lots of profit out of the deal.
In order to gain favour with the Queen of Throng, Bob reverses the direction that Ahead of Under was moving and thus ends the motion sickness experienced by the continent's populace over the years. Unfortunately this sent the land mass on a collision course with Under, and Russel has to travel to Ahead of Under and try to set things right before his kingdom's crushed. Meanwhile Vernon and Populah have to protect Under from the ravenous Bedrock Beasts emerging from Ahead of Under.
Russel, Gus and Loopy decide to go on vacation on Frolicking Island, only to find it is not a resort, but a horrible trap ruled over by Happy Fernando and his pain-inducing whistle. Russel works with Captain Darling (who is also trapped with Auntie First on the island) to try to escape. Meanwhile, Vernon creates an Unmeltable Ice Cube that turns all liquid it touches solid, but when he tries to dispose of the dangerous invention, Bob Wire steals it to set up a bogus sales campaign.
Bob's advertisement campaign for the metal polish Kleng has everyone in Under and Near Under mistakenly awaiting an attack by Kleng, believing it to be a fearsome conqueror. When Auntie quickly surrenders her country to Bob before "Kleng" comes, he quickly takes the opportunity to trick Russel too for his crown too. Russel becomes suspicious and tries to find the truth about Kleng.
When Under's Squump population gets too old and retires, the irritating Slaphappies move in, forcing Russel to go to the Argyle Flats and rope up another herd, a task only possible by subduing the biggest bull Squump. Bob Wire is also out to get the Squumps, and hires some local help to do it.
Desperate to get Bob out of his funk, the Frags go to the shady swindler Thornhop. He agrees to get the crown and remove Russel from the picture in exchange for Bob's beautiful (singing) voice and the Frags agree. Thornhop installs an unbreakable latch on the portal to Up and refuses to let Russel return home unless he handed over the crown. Bob tries to go back on the deal, but Thornhop steals the new King's voice. Meanwhile, Russel tries to crack the combination lock from his side of the latch.
Cliff gets Russel to go to Under and recover his memory of Grade 4, which he forgot after falling into the GeeIForget River on the same day he had hidden the second Crown of Under. Bob was also looking for the second Crown, and decides to get Cliff’s memory so he could find the Crown and get 50% control over Under. Unable to steal it from the Recollecter at the river's end, Bob then decides to get Russel to get the memory instead then he’d steal it from him. But he remembered that he had set traps all along the river bank to stop Russel, and one of them knocks Russel into the river, making him forget everything.
When a giant Asymptote appears but stops short of the town, the Underites start panicking, and Vernon goes dangerously Babbleyboing. In order to stop him from exploding, Russel takes him to the Clockmaker, who now lives in the Bottomless Pit. Unfortunately, the Clockmaker gave all his belongings to his love Flavia, and as such will be unable to fix Vernon until he marries her. Meanwhile, Loopy and Gus try to get rid of the Asymptote, with Bob (who is making a killing in the therapeutic business) fouling them up at every turn.
As an alien fleet approaches Under, Russel is arrested by Agent Greg and his Future Infractions Bureau for a crime he has yet to commit. The others head to the Museum of Tomorrow to try to find out what crime he will commit, but are arrested themselves for planning on breaking Russel out of jail. Gus has everyone speak in past tense instead of future tense so they won’t be caught again, and he and Russel escape with the help of Vernon, Loopy and Populah, and head to the Museum to visit the future.
When Russel overhears that his mother needs money and is going to sell the house, he goes to Under to find some treasure for her instead. He and his friends head out to collect the Orb of the Zinkas, an extinct sugar-loving culture. Bob tries to stop Russel, believing if Under got a new King, he’d stand a better chance of stealing the Crown.
During the live production of Bob’s new TV talk show, Auntie’s prized Charismango Tree disappears and she wrongly accuses her new Thorch of eating it. Russell decides to save the Thorch and bring it back to its forest home, but Auntie hires the famous tracker Mr. Queep to get it back. Meanwhile, Vernon and Loopy try to find the real culprit, with Bob, the Frags, Hugh the Yu-Yu and Cousin Tess as their main suspects.
Loopy "trespasses" in Near Under while picking flowers, causing Auntie to declare war. Russel calls for a truce but when they meet at the peace talk, Bob has them eaten by a Gurd, which turns them into eggs. When Russel hatches he becomes obsessed with getting to the top of Unclimbabale Mountain, but the Troll who owns it tries to stop him. When Auntie hatches she becomes infatuated with Loopy and puts him in a cage, he then asks her to get the Crown for him, since he saw that Bob had stolen it from the preoccupied Russel.
The Frags are forced to join Loopy as Gopher scouts for their community service hours and together they steal the last Thussle bush, which had been causing a war (both the Red and Yellow Floronians wanted it). They give it to Bob Wire to keep safe, and he tells the Floronians that Russell has it causing them to attack Under. Bob tells Russell he will give it back and end the siege if he gets the crown, however Captain Darling decides to use the Thussle as a Despot Day gift to Auntie First.
Scared of a storm, Gus has Russell go down to Under where the Horrorborealis is filling the skies, turning everyone organic (i.e. not Vernon and Bob) into pickle-obsessed Ghouls. Russell, Gus and Bob gets hit by lightning and are negatively charged, protecting them from everything even turning into Ghouls. Russell decided to go to the West Pole where his charge will be neutralized, and end the Horrorborelais as well, allowing everyone to return to normal. Bob, however, gets himself positively charged and threatens to neutralize Russell himself unless he gets the crown, but Gus ends up turning into a Ghoul. Meanwhile back in Under, Vernon has to eat all the pickles so the Ghouls can't themselves, as that would make them remain in their monstrous state forever.
Bob kidnaps Gus and Loopy, sending them floating away to Dessert Island where they are beset by ravenous Sea Scarfs. He then has a Shapeshifter turn into Gus to steal the Crown, but the plan is disrupted when Russel and the Shapeshifter have to save Vernon from a nest of baby Geets.
The Kingdom inspector is making his rounds and Vernon, Populah and Loopy are cleaning up Under in preparation, kicking everyone out. If the kingdom’s dirty, then Russel will lose his crown, so Bob sets out to turn the town into a dump. Unknowingly Auntie First uses an obscure legal loophole to swap ownerships of hers and Russel’s kingdoms, leaving Vernon and Loopy to clean up filthy Near Under in a few mere hours. Meanwhile, Russel, Gus and Populah have to get the last stamp required in Under’s Stamp Collection from Elpsun C's post office (without which he will lose his crown also).
Auntie gets a Spitesucker stuck to her face when she visits Bob’s Aqua Zoo, forcing Captain Darling to make a deal with Cousin Tess to get it off. Unfortunately once she was freed, Auntie couldn't remember being sucked and was forced to try Darling and Tess for treason. Meanwhile, Russel also visits Bob’s Zoo and gets attacked by a Plurn. Gus saves Russel but the Plurn inks him, turning him into an aquatic animal. Russel goes to ask Tess how he can get Gus back to normal (leaving the finned dog with Bob and Loopy, who put him on show), but Tess refuses to help Russel until he wins her and Darling’s trial.
Loopy accidentally activates the jetpack Vernon was making Russel and rockets away to a distant village of Iples who dub him their "Great Decider" (they can't make decisions themselves). Meanwhile, Populah’s hair has been turned into snakes by Auntie’s shampoo, and Auntie won't share the antidote unless Russel gets her a date with Mr. Queep. Queep agrees to go on the date once he has captured a Nonifer up in the monster-infested NotVeryKnown River, but his canoe sinks at port. Gus volunteers Russel’s King Ship and they accompany Mr. Queep on his quest, along with Bob and the Frags, looking for a story to sell their tabloid, the Hotwire
Bob Wire releases Gag Pune from Mount Stoney Lonesome, he was imprisoned in when Cliff was King. Gag was imprisoned in the mountain after going on a rampage after being laughed at by Cliff, Bob and the inhabitants of Under during a festival. Bob freed Gag because he knows the first thing he'll do is head to Under to destroy the place and Russel will have no choice but to get Cliff's help, and the price will be the Crown of Under. Meanwhile, Loopy needs to get his nose bell fixed and we learn that he's more than just the court jester.