Andy and Terry live in a 91-story treehouse. (It used to be a 78-story treehouse, but they keep getting ideas for new stories!) It has a submarine sandwich shop that serves sandwiches the size of actual submarines, an air-traffic control tower, a human pinball machine, a spin-and-win prize wheel, a giant spider web--with a giant spider!-- and a big red button, which they're not sure whether to push or not because they can't remember what it does.
Good thing there's so many fun things to do in the treehouse, because Andy and Terry get stuck babysitting Mr. Big Nose's three grandchildren for the day.
After all, how much trouble could they possibly get into in just one day?