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...

Cassie and the other Animorphs have another alien invasion to deal with, the Helmacrons. They are fierce, war-like, intent on taking over the earth and only one and a half millimetres long. But they have a dangerous weapon - a shrinking ray.

Someone's looking for Tobias. Someone who says she's his long-lost cousin. Tobias isn't sure the person is telling the truth, but she's really nice, and knows a lot about him. And what she tells Tobias definitely gets his attention.It seems a lawyer has discovered Tobias's father's last will and testament. So, Tobias needs to attend the reading. His cousin even offers to go along. But something just doesn't feel quite right. That's when Tobias and the other Animorphs decide to do a little checking on this "cousin." And what they discover will change the rest of Tobias's life...

The Animorphs have no ally more powerful than Erek the Chee. But now he is helpless.

Marco and the other Animorphs have managed to find out where the Yeerks are planning to build their next ground-based Kandrona. That's a good thing. But the location is supposed to be somewhere near the North Pole. That's a bad thing. The Animorphs know that the Yeerks are a "cold-blooded" species, but this is a little nuts! Who wants to be near the North Pole without Arctic morphs -- and wearing spandex?Even so, the kids know that if the Yeerks succeed with their plan, Earth is pretty much done for. And Marco and the other Animorphs aren't quite ready to give up the fight.

Jake is just a normal kid. Well, as normal as possible considering he can morph animals, and he's in a war against parasitic aliens. But as unbelievable as it sounds, something even stranger has happened. One morning Jake wakes up, and he's twenty-five years old.Okay. Maybe it's a nightmare. Or maybe Jake's just lost it for a while and misplaced a few years. And there's another problem. The world Jake-the-kid went to sleep in has changed. It's ruled by the Yeerks. Jake has to find out if the other Animorphs are still around. Still somehow fighting. Or if he's really on his own...