Underground

The Animorphs try out the disk, but nothing happens. Jake and Marco realize that someone very close to Jake is a Controller. The Controller turns out to be Jake's brother Tom. Jake wants his brother back, and he'll do anything to do it. They tried morphing out into ferocious animals from the clinic that Cassie's mother works in. Jake acquires a Tiger and Rachel acquires a Lion. They use it to try and free all of the humans in the Yeerk Pool located under the school. Their attempt ended with no free humans, without Tom, Tobias was gone (Probably captured by the Yeerks), and the disk was lost.