Gregor, whose father went missing several years before, is babysitting his little sister when she opens a grate in the basement floor and falls into the Underland, far below the Earth's surface. There he meets rats, bats, beetles, spiders, all of which are man-sized, and men who traveled down below the surface hundreds of years before. He tries to leave and fails, but hears word of his father. He then sets off in an adventure that, hopefully, will allow him to save his father and all the other humans in the Underland.