Finding a weird metal cylinder in the junkyard—that's where it all started for eleven-year-old Harper Graybridge. The young explorer had no idea his discovery would lead to an encounter with a three-foot-tall bug-man named Lyt. When Lyt reveals he is searching for the very same cylinder—known to him as a keeper—Harper realizes his discovery is the key to an adventure of a lifetime. A bargain is made, and Lyt agrees to be Harper's guide into his world, the underground realm of Murkmor.

It doesn't take long for Harper to find out Murkmor is as scary as it is wondrous. Running from poisonous jelly-monkeys, navigating treacherous ruins, and sliding down the inside of a giant worm are just a few obstacles Harper must overcome. Thankfully, Crowll, a bird-like warrior, has pledged to protect Harper on his journey.

Harper, Lyt, and Crowll attempt to avoid danger and keep a low profile. Unfortunately, word of Harper's appearance reaches Baron Vyndot, an evil overlord who believes the keeper is vital to uncovering a long-forgotten legend. With a lot of courage, some quick thinking, and the help of some bizarre new friends, Harper may be able to survive and escape this crazy underground world. Returning home to his family will be complicated—returning home unscathed will be impossible.