Almost nothing could be as bad as finding out your mother is
Visser One. The most powerful of all Vissers. The leader of
the Yeerk invasion of Earth. But it happened to Marco. And
even though he's been handling it pretty well, he knew there'd
come a time when he'd have to face her again. Knowing that
the Yeerk in her brain had taken his mother away.

So when Marco, the other Animorphs, and Ax discover that
Visser One is overseeing a secret underwater project, they
know they have to check it out. But Marco's not sure if this is a
battle he'll be able to fight...