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 are not only faced with trying to slow down the Yeerks' invasion, but they also have to 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...