When the Wright family moves into their new home, Russel is surprised to find a secret passage under his bed. This passage leads him and his dog Gus to the world of Under, where he discovers he is the new King and that Gus can talk! Russel and Gus quickly befriend the people and creatures of this mysterious land. Bob Wire and his evil team are continually conspiring to steal Russel's crown and become all-powerful rulers of the land.

It’s the Adventure Bay 500! The pups have built an awesome race track and are ready to be the pit crew for their race hero, The Whoosh! But when the legendary racer is unable to drive in the championship race, he calls on his biggest fan-pup Marshall to take the wheel and race in his place! Marshall has to overcome his lack of confidence and his dastardly competition, The Cheetah, to fulfill his dream of becoming the fastest race-pup ever!