When eleven-year-old Gregor and his little sister Boots fall through a grate in the laundry room of their New York City apartment, they hurtle into an underground world they never knew existed. Fourteen-year-old Belle meets the pair in her home, the Underland, where humans live uneasily beside giant spiders, bats, cockroaches, and rats. But their fragile peace is about to fall apart. Belle and Gregor must take part in the Prophecy of Grey to stop a horrific war, and save the Underland.