Summary
Spoiler:
After losing control during a battle and needlessly killing a Hork-Bajir, Cassie decides she wants to quit the Animorphs. The others are angry, but Cassie is stubborn.
A day or so later, Cassie is riding a horse when she sees a little girl running from a bear. She saves the girl, but is forced to jump into a river to get away. She loses consciousness and wakes up downstream, completely lost. The little girl, named Karen, has twisted her ankle but is otherwise okay.
Well, sort of. It becomes evident in about two minutes that Karen is a Controller. Her Yeerk, Aftran, followed Cassie home after the battle because the Hork-Bajir Cassie killed was Aftran's brother's host. Cassie pretends to have no idea what Karen/Aftran is talking about and focuses on getting them some shelter. When an escaped leopard attacks Karen, though, Cassie is forced to morph to save her. Aftran is just happy they can stop pretending now.
Cassie and Aftran discuss some interesting aspects of Yeerk life, though the conversation ends in a fight. But Aftran is beginning to crumble, and says that she won't leave her host and spend eternity as a blind slug unless Cassie agrees to do the same. So Cassie traps herself in caterpillar morph before the other Animorphs can stop her.
The others want to rip Karen's head off, but Aftran convinces them that she and Cassie had a deal. So, heartbroken, the Animorphs take Cassie home. She spins a cocoon and emerges as a butterfly a few days later. At which point Ax says that the morphing process should have reset, since she switched forms, and she has two hours to demorph.