Russell Wright, his dog Gus and his family move into their new home in a new neighborhood. Russell happily chooses the turret room for his bedroom, where he finds a secret passage under his bed. The passage leads him and Gus to the world of Under where he finds out that he is their new King and that Gus can talk! There is however a little snag, he wont officially be the new king until he rescues Vernon from the room you cant get out of and snags the crown from the nasty Bob Wire. When Russell finally meets up with Bob Wire he is challenged to a duel for the crown. In a strange Underesque battle, Russell defeats Bob and becomes the new king of Under King Russell the Wright! Meanwhile in the Up world a mean kid named Cliff makes claims to the turret room, which used to be his. It seems that Russell will be defending his crown in both worlds.

Russell and Gus find Vernon and Loopy arguing over an Urklet egg that Loopy insists on keeping. Russell receives mail from a dart bird stating that Ex-Princess Populah is in trouble and needs him to rescue her. Russell, up for the challenge, takes Gus and Vernon with him while Loopy stays behind to take care of his egg. Upon reaching Populahs tree house, they realize that the letter was another ploy by Bob Wire to snag the crown. Russell and the Gang take off with Bob Wire in hot pursuit. Our heroes make it under the Floating Mountains just before they crash to the ground. Bob and the Frags arent so lucky. Back at the castle, Loopys egg hatches and a terrible creature called a Rebulok is born not the harmless Urklet that Loopy was expecting! The Rebelok escapes and heads to Up where it runs into Cliff who is trying to get to Under. When the gang gets back to the castle to find the place a complete mess from the Rebelok running around. Russell and Gus leave Vernon and Loopy to clean up the mess on

Loopy and Vernon get lost in Near Under where they run into Auntie First who locks Loopy up in her Pet Sanctuary. Back in Under, Russell explains to Vernon that they need money for a ransom to get Loopy back. Vernon doesnt know what money is and Russell realizes that they use a barter system in Under. Russell tries to explain what money is and lists different names used for it. Vernon recognizes the word Greenbacks so Russell sends Vernon off to get some. What Russell doesnt know is that in Under Greenbacks are frog-like animals. Bob Wire hears about the plan and decides to corner the market and capture all the Greenbacks in Under. In Near Under, Russell and Gus decide to confront Auntie First and her sidekick Captain Darling, who then order Gus thrown into the pet sanctuary with Loopy. After some arguing Russell convinces Auntie that they should have a vote. Unfortunately, in Near Under everyone votes for whatever Auntie First wants. Russell and Populah plan a daring escape and free all of the anim

Russell is having a bad day because of Cliff and decides to head for Under. Getting there he sees that Loopy and Vernon have designed a car for him. Loopy decides to do Russells homework for him and gets the Underites to help him. Russell, Gus, Populah and Vernon go for a ride in the car. Bob Wire is talking with Auntie First and deciding that they need a secret weapon. Bob Wire sees Russells car and comes up with a plan to steal it. He is successful and brings it to Auntie First. She orders to have it taken apart so that they can make a bajillion of them. Russell and the gang go after Bob but Auntie First captures them and orders Vernon to tell them how to make the car. Russell, Gus and Populah escape, grab Vernon and get back to the car. Russell realizes the only way to get Auntie and Bob Wire off his back is to destroy the car. Once they demolish it by letting it go into a canyon they head back to Under. Loopy is busy trying to solve a math problem and at the same time is flooding Under with one

In Under Vernon is tweaking a Riftulator but its shaking everything in Under. They realize that creating a rift around Under will let out all the Night Slythers which would cause a huge mess so they decide to close the rift and hide the key. Bob Wire and the Frags hear this plan and decide to steal the key. Meanwhile, Russell overhears Underites blaming their problems on a Pudrick, which is blamed for everything bad that happens in Under. Russell sees that the Pudrick is a small furry cuddly creature and orders them to stop. Russell orders Loopy to stop the Underites from blaming the Pudrick. Unfortunately, when you dont blame a Pudrick it grows into a monster and keeps growing until it is blamed down to size. Russell and the gang catch up with Bob Wire and realize that Bob has started up the Riftulator and hidden the key. He agrees to give Russell the key for his crown, Russell of course says no. They all hear a loud noise and Vernon explains that is an unblamed Pudrick. Gus goes with Vernon to fin

Russell has been feeding a stray dog and the howling from the dog is keeping the entire neighbourhood up. He decides that he wont be getting much sleep so he heads for Under with Gus. Once they arrive in Under they find Loopy and Vernon tied down. They explain that it is Trash Night and that he and Gus should tie themselves down also before the moon sweeps down and it gravitational pull takes them up with the trash. But it is too late and Gus, along with Russells crown, ends up on the moon. Bob Wire sees what has happened follows. Vernon and Loopy explain to Russell that the moon makes 3 passes and once it the third pass comes it will be gone for a whole year. Russell and Populah head to the moon on the second pass. On the moon Gus meets the Hermit and quickly discovers the Hermit is lonely and likes to talk a lot. Once Russell and Populah land on the moon they find Gus but also learn that the moon cannot hold any more trash and that it will crash into Under next year if the Underites dont stop the T

When Russell arrives in Under he notices that everyone is sleeping. He discovers there is a plant that produces pink pollen, which puts everyone to sleep. He decides to attach the plant to a balloon and send it into the sky, waking everyone up. The Underites fill him on the once a year event called the Pollen Holiday. That same day they discover some unknown monsters emerging from the Sassafraz Sea and corpula vines are suddenly taking over the Flatulent Swamp. The monsters begin to eat the corpula vines and the gang also notices nests of baby monsters. As Russell and the gang watch they realize that the reason they have never seen these monsters before is because the pink pollen plant puts them to sleep so that nobody interferes with the life cycle of the monsters. Russell gets the pollen plant back and puts everyone to sleep again. Even in Under all life is strangely connected.

Russell gets Vernon to make him a roller coaster. Bob Wire stirs up trouble between Auntie First and Russell. His plan is to have them fighting so he can take over both countries. Auntie First starts by putting up a huge wall in between Under and Near Under to block out Russells roller coaster. The wall ends up restricting the Gorble Birds from flying around Near Under and they end up perching on the wall and creating a mess. While Vernon and Loopy trying to remove the birds, Russell, Gus and Populah head to Near Under to ask Auntie First to remove the wall. With no luck they decide to return to Under only to find that the wall has been removed. Thinking it was Auntie who relented and tore down the wall, they continue with the plans with the roller coaster. Auntie First is furious as she thinks it is Russell who is responsible for tearing the wall down. Gus ends up tricking Bob Wire in revealing his plan to get Auntie First and Russell mad at each other. All is revealed and everything returns to no

Auntie First has schemed a plan to have everyone in Under wear Auntie pants, once any water hits the pants they go solid like cement and allows her to take over in Under. Russell gives Gus a bath in Under because he has flees resulting in Russells crown being sucked down the drain. Bob Wire discovers the crown and goes after it. Vernon stays in Under to clean up all the fleas, while Russell, Gus, Loopy and Populah decide to go after the crown and find themselves in Under Under. There they meet Hugh the Yu-Yu and Mavis the night Slyther. After hearing Bob Wire and Russell fighting over who is the real king, Hugh decides that the last king standing in a fight against Mavis will be awarded the crown. During the duel, Russell discovers that Mavis has his crown stuck in her throat, saves her life and wins the crown. They head back to Under to discover Vernon has all the Underites bathing to get rid of the fleas as a have to take off the Auntie Pants, thereby foiling Aunties plans of domination.

Russell is practicing for his Gopher Scout merit badge and decides to head to Under to practice. Vernon decides to help Russell practice for his badge in the Quithering Forest but instead of roughing it, Vernon turns the forest into a spa for. In the Quithering Forest, Bob Wire and the Frags manage to capture Russell and the crown. Back in Under, Auntie First tries to run a bluff, convincing Loopy, Gus and Populah that she has an army and is going take over Under. Without their King to guide them, they decide to run their own bluff and convince Auntie that they have an even bigger army. Using the Gopher Scout Guide Book, Russell escapes Bob Wire and retrieves his crown.

Vernon and Loopy know that Russell is unsuccessful when trying out for the football team in Up so when he arrives in Under they announce that he has made the joustabout team. Vernon and Loopy make it the law that Russell is to always win. Bob Wire hears about the law and decides to place bets with the Underites where he will always win the bet. Sure enough Russell starts winning all the games. Populah catches on and tries to tell Russell but Vernon and Loopy intercept and stick her in the room of confusion where she cannot tell Russell the plan. Because of his confidence Russell ends up challenging Bob Wire and they bet the crown. During the game Populah gets free and tells Russell that Loopy and Vernon were behind him winning. In the meantime The Frags try to rig the game in Bobs favor, but end up making matters worst and ultimately costing him the game. Russell keeps his crown but decides that joustabout is not his game.

Russell dresses up as his comic superhero, the Coyote, and heads to Under to help fight crime. While Russell is fighting crime the Coyotes enemy, the Grimster shows up and at the same time Russell accidentally reveals to Bob Wire that he is fact the Coyote and without the crown. Bob Wire hatches a plan to snatch the crown when Russell is dressed up as the Coyote. When the Coyote tracks down the Grimster it is revealed that Loopy has been dressing up as the Grimster so that Russell will have a real comic book villain to fight. Bob Wire changes his mind about his original plan to steal the crown and decides instead to take over as the Grimster. He finds a huge mutant leech to destroy Under. Russell battles with the decision of whether to be the King or the Coyote. He chooses to be King, realizing that you can be a hero with the cape. The leech is returned to a smaller normal size and Bob Wire is revealed as the Grimster. Russell saves the day once again.

Loopy accidentally turns the castle into pumpernickel with one of Vernons gizmets. Upset about his blunder, Loopy runs away to join the Pirates of Hole Island. Using the pumpernickel castle to his advantage, Bob Wire tells Russell he wants the crown or else his rock monkeys will eat the entire castle. Vernon stays at the castle to try to get things back to normal and to deal with Bob Wire while Russell, Gus and Populah try to find Loopy. Unfortunately before they can get to Loopy they are captured by the Pirates themselves and instructed to dig for treasure. Loopy is on the pirate ship but doesnt want to return home to face the face what he has done. Russell thinks of a plan to outwit the pirates and convinces Loopy to come home with them. Bob Wires rock monkeys all take a bite out of the castle just as Loopy turns the castle back to normal.