Mo, a young girl who can't talk, and Duke, a very curious dolphin, communicate in ways that go far beyond words. They are asked to help when an explosion opens the cavern of intelligent and technologically advanced octopus-people. The cavern is near a secret underwater garden so rich in precious metals that it attracts attention.

Mo and Duke are called on to build a bridge of friendship between the underwater aliens and humans - a disastrous rise in sea-level lies in the balance.