The stranger / Applegate, Katherine

Rachel and the other Animorphs have finally found the new entrance to the Yeerk pool and have figured out how to sneak in. But things get complicated, particularly when they get a chance to decide if they want to stay on Earth or go to another planet.

Rachel and her fellow Animorphs believe they have a real chance at defeating the Yeerks when they discover the location of the alien regeneration pool, but their plans are disrupted by an all-powerful Ellimist who offers them an opportunity to go live on another planet.