Arthur and Will Denision continue their adventures on the strange and wonderful island of Dinotopia. When Will accidentally discovers an ancient temple ruin guarded by not very friendly tyrannosaurs...he barely escapes their snapping jaws.

Meanwhile, Arthur leads an expedition deep into the mysterious caverns of the World Beneath. They discover a fleet of "strutters," strange mechanical walking machines shaped like dinosaurs and powered by sunstones. But the harmony of the entire island is threatened when a member of his expedition attempts to steal the Treasure of Posiedos, angering not only the tyrannosaurs, but also the even larger and fiercer meat-eating Giganotosaurus!