When Uncle Bernie turns up with his incredible Mechanical Mole machine, the Adventuremice go on a mission! Pedro and the crew burrow to an underground cave world, unlike anything they've ever seen. There are tiny, twisty trees, and even tinier pterodactyls! But there's also a HAMSTERSAURUS REX on the loose! Can the team save the nervous local cavemice from this giant predator, or is there more to the creature than meets the eye?