Marco's mom is back. But she's not Visser One anymore. Marco's not even sure if she's still a Controller. But he's determined to find out. No matter what it takes. No matter what might happen. Marco wants his mom back. Jake, the other Animorphs, and Ax realize that Marco is under some serious stress. And that the situation with his mom could very well jeopardize everything they've worked for. Now they also have to wonder if Marco will be the one to give away the secret of the Animorphs...
Something is very wrong with Ax. He's sick. And the Animorphs don't know what to do about it. They can't take him to the hospital. They don't know how to contact the Andalite home world. And Ax is dying. Cassie and the other Animorphs come up with a plan. But it involves finding a new way into the Yeerk pool. There's just one problem: Jake, Rachel, Tobias, and Marco come down with Ax's sickness. And now, for the first time, Cassie is on her own...
The Animorphs have met the Ellimist. He "helped" to save the kids when they were about to be eaten by a Taxxon. He "helped" to free two Hork-Bajir and restored Tobias's morphing ability. But, even though the Ellimist has enormous power, he is not all-powerful. He has an enemy. The Crayak. So, the Crayak and the Ellimist decide that a battle will prove their ultimate power. But they don't intend to fight each other. The Ellimist will choose the Animorphs, Ax, and Erek, the Chee; the Crayak will choose his own army. But if the Animorphs lose they will be erased from the universe altogether. And there'll be no one left to fight the Yeerks...
The Animorphs and Ax have one true ally in their battle against the Yeerks: 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. And the Animorphs realize what will happen if Visser Three discovers the valuable secrets of the Chee. One of which is the Animorphs...
The Yeerks have decided to try a little experiment. They've decided that if they can develop a way to make people more "willing" to be infested, they can speed up Earth's invasion. So, the Yeerks try to invent a substance that will take away the human ability to make a decision. A substance that will eliminate free will. Now the Animorphs and Ax are not only faced with trying to slow down the Yeerks' invasion, but they also have to try and put an end to the genetic testing on a small group of humans. This time there's only one chance to stop the Yeerks. And if anything goes wrong, it's all over...
