The newest Animorph may prove a greater threat to the team than the Yeerks who are again threatening the world.

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.

Jake has discovered that there may be other people who know about the invasion. He has even discovered a Web site devoted to the Yeerks, but he has to be careful about investigating it.

Cassie believes her strange dreams are just a side effect of having been given the alien power to morph into an animal, but when she learns that Tobias has been having the same dreams about the ocean, they realize someone is issuing a call for help.

The Animorphs have one true ally in their battle against the Yeerks: the sentient androids known as the Chee. So, when they discover that the Chee - including their friend Erek - are in trouble, they have no choice but to help. But the Chee's problem is much deeper than anything the kids could have imagined. It's hidden in the ocean. So far down that no human has ever travelled there and survived. The Animorphs realize what will happen if Visser...

The Yeerks are working on a drug that saps humans of their free will but the Animorphs show them that human free will runs deeper than any drug can reach.