Kara and Joe spend their days navigating the perilous waterways of a sunken city, scratching out a living in the ruins. But when they come into possession of a mysterious map, they find themselves in a world of trouble.

Suddenly everyone s after them: gangsters, cops and ruthless Mariner pirates in their hi-tech submarines. The two children must find a way to fight back before Floodworld s walls come tumbling down…