Eleven year old Gregor is stuck at home with his poor grandmother and baby sister instead of going to summer camp like every normal kid. His mother has to go to work, as his father disappeared just over 2 years ago. He is stuck babysitting his sister and his slightly delusional grandmother, when he falls through a grate in the laundry room of his New York City apartment building. With his two-year-old sister, Boots (also known as Margaret), he lands in the Underland. Here giant spiders, rats, and cockroaches coexist uneasily with humans. Because a prophecy foretells that an Overlander will play an important role in the uncertain future of Underland (which is on the brink of war), Gregor's arrival is treated as no accident. When he learns his father's disappearance may have something to do with the Underland, Gregor reluctantly embarks on a life-changing adventure.