While Jake and the other Animorphs try to stop Visser Three from reaching the Pemalite ship at the bottom of the ocean, they discover a strange underwater civilization.

A boy named Jake wakes up to find himself twenty-five years old and in a world ruled by the aliens known as the Yeerks.

Cassie didn't pay much attention to the dreams. She and her friends have been having nightmares ever since they acquired the power to morph. But when Cassie discovers that Tobias has been having the exact same dreams about the ocean, and a voice that's calling to them for help, she decides it's time to start listening.