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