The Hidden (Animorphs #39)

We can't tell you who we are. Or where we live. It's too risky, and we've got to be careful. Really careful. So we don't trust anyone. Because if they find us...well, we just won't let them find us. The thing you should know is that everyone is in really big trouble. Yeah. Even you.

The pint-sized Helmacrons may have departed Earth, but they haven't finished causing trouble for the Animorphs and Ax. They left behind a damaged ship–containing morph-tracking technology. Now the Yeerks have found the ship, and are launching a full scale Animorphs hunt!

As if avoiding the Yeerks' new tracking system isn't hard enough, Cassie and the others realize they must keep the morphing cube moving if they don't want the Yeerks to find it. But when Cassie tries to hide the Andalites' cube, it accidentally touches a Cape Buffalo! Now there's a mean old buffalo with morphing powers on the loose. And he could give away the secret of the Animorphs if he charges into the wrong hands....