There's a new rumor in town. Someone has discovered an
item that proves life on other planets exists. And they've been
hiding it on a base called Zone 91. The Most Secret Place On
Earth.

Cassie, the other Animorphs, and Ax already know about life
on other planets. Too well. They also realize the Yeerks will try
to access Zone 91. To find out if what's there will threaten
their mission. So the Animorphs decide to pay Zone 91 and
the Yeerks a little visit. But what they discover is not at all what
they expect...