One day, Isaac Thompson is just your average fifth grader playing the part of a porcupine in the school play. The next, he has strayed into a mysterious subterranean realm that has been lurking beneath his school Castle Elementary and launches his quest to knighthood. When Isaac's little sister Lily goes missing from their school's creepy basement, he and his best friends Max and Emma set out in search of her. Their search takes them to the Underground, where they encounter an army of spear-wielding rats, a talking human-sized bat, and a thumb-nosed prison guard. But humans who stay in the Underground too long transform into weird, unpleasant creatures and are forced to work for the horrible Elf King. Can Isaac and his crew escape the Underground before it's too late for them to ever return home?