Rachel is still reeling from the news that the Earth is secretly under attack by parasitic aliens known as the Yeerks. And that she and her friends -- five kids who stumbled onto a downed spacecraft and were given the power to morph into any animal they touch -- are the planet's only defense. But Rachel has always been a daredevil. When it's suggested that they infiltrate the home of their assistant principal, who also happens to be a human host of the Yeerks, she volunteers. But what she finds inside may be more than even she can handle.