Harold, a brainy sophisticated ten year old from London, and Agatha, an adventurous, rambunctious eleven year old from California, are total opposites. They have recently become stepbrother and sister, and are not very happy about it. When Agatha is forced to take Harold on a hiking trip, they discover a mysterious jewel, unlike anything they've ever seen, and decide to go back and look for more. Unfortunately, a strong earthquake plunges them down into a subterranean world where an evil king has enslaved a once peaceful race. Now, Harold and Agatha must risk everything to help their new friend, Princess Qubar save her people from the tyranny of King Zantar, and then get back home before their parents return from their honeymoon.