As Andalite is dying, he gives Jake, Cassie, Marco, Tobias, and Rachel the power to "morph"--to transform into some of the most dangerous creatures on Earth, in an attempt to save humanity from a silent attack.

Jake and the other Animorphs are ready for a new battle in their war against the Yeerks. They reckon they know where their enemies' new base is--and they've even worked out how to get in. Nobody's going to notice a few flies on the wall, are they? But things go wrong and their worst nightmares come true when a Yeerk crawls into Jake's brain. And now it's controlling him Using him. And learning all the Animorphs secrets.