The Threat (Animorphs #21)

There is a new Animorph. And he's arrived just in time, because the Yeerks are preparing their biggest takeover ever. Their ultimate target: the world's most powerful leaders, all gathered together in one place. What better way to get into the minds of humans? Literally.

At first, David joins the fight with a vengeance. But there's definitely something wrong. Because he's starting to break the rules, taking risks that could get them all captured. Or killed. The Animorphs don't know what to do. There was a time when the Yeerks were their greatest enemy, but that's about to change...