There's only one passage between the Upper World and Droon — the magical rainbow staircase. At least, that's what Eric and his friends have always believed. But now Ko and his beasts are attacking Zorfendorf Castle, a magical fortress that's hiding a huge secret: the Fifth River, another passage to the Upper World. Ko will do anything to get to the river — and to the Upper World. And Eric, Julie, Neal, and Keeah will do anything to stop him!