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. The ultimate target: the world's most powerful leaders 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 is 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. Because there was a time when the Yeerks were their greatest enemy. That's about to change. . . .