In a world of talking animals, there is one burning question: Did humans ever exist? Thelonious Chipmunk believes he has proof that they did: a postcard showing a building made of concrete and glass. But his sister thinks the postcard's just a dumb piece of paper, and that humans are only make-believe.

Then one day a flash flood carries Thelonious away. When he comes to, he's in a strange world that looks something like his postcard, but not quite. Then he realizes that this is the city on his postcard, only crumbling and decaying. But if this means humans did exist, where did they go? Thelonious, along with some new friends, sets off to find out.

And thus, the adventure begins!