The Illusion

In this book, the character Tobias is caught in a perilous situation when he is given a mission to infiltrate a Yeerk gathering. He's going to be tested in ways he could never have predicted, and morphing into a human no longer guarantees his safety. Tobias and the other Animorphs already have a few problems: saving the world, keeping their identities secret, finishing their homework. Now they have one more thing to add to the list. The Yeerks have discovered a way to cause a person to demorph. And with this new device Visser Three plans on capturing 'the Andalite bandits.' The Animorphs realize they have to get rid of the device -- or be captured by the Yeerks. And the risk of getting caught is very high. That's when Tobias comes up with the only possible plan. Someone has to acquire Ax.