- Narrator: Cassie, Jake
Cassie quits the Animorphs after she kills a Hork-Bajir Controller and her father's veterinary clinic nearly gets shut down.
Tropes:
- 10-Minute Retirement: The stress of the war catches up to Cassie and she quits the team, telling the others that the Wildlife Rehabilitation Clinic will be shut down soon and they can't meet there anymore.
- And Now for Someone Completely Different: The book switches to Jake's point of view after Cassie morphs into a caterpillar.
- Appeal to Inherent Nature: Aftran defends the Yeerks' parasitic conquest by comparing it to human predation of cows and pigs.
- Enemy Mine: Cassie and Aftran are lost in the woods with an escaped leopard on the loose. They have to work together to survive, even after Aftran realizes that Cassie is an "Andalite bandit".
- Honor Before Reason: Rather than let Marco kill Karen to kill Aftran, Cassie instead allows Aftran to infest her, giving away all the Animorph's secrets to a Yeerk.
- Jerkass Ball: Marco is written as particularly cruel in this book. Special notes go to him mocking Cassie about having torn out a Hork-Bajir's throat like it's the first time the Animorphs have knowingly killed someone, and immediately trying to kill Karen instead of the more logical route of starving the Yeerk out of her, because somehow a Controller with a five-year-old host body is that much of a threat.
- Kill the Host Body: Discussed, though not for the first time. The Animorphs have killed Controllers before, usually only sparing their hosts pitying thoughts, but the realization that they do this drives Cassie to abandon the Animorph's mission.
- My Species Doth Protest Too Much: What allows the plot of this book to work out for Cassie is that Aftran is as much of a pacifist as she is.
- Reckless Pacifist: Cassie refuses to kill Aftran because this somehow means killing Karen too, never mind that Karen is a five-year-old girl easily overpowered by any individual Animorph, and they have allies like the free Hork-Bajir and the Chee who could easily contain up to an adult Hork-Bajir Controller for however long it takes to starve the Yeerk. She allows Aftran to infest her instead, putting her fellow Animorphs, their families, the free Hork-Bajir, and the Chee all at risk of Yeerk enslavement or extermination.
- Shapeshifter Mode Lock: Cassie traps herself in caterpillar morph to prove to Aftran about how serious she is about empathizing with the Yeerks. Unlike most nothlits, the caterpillar has a way out...
- Status Quo Is God: Natural metamorphosis can reset the morphing clock, allowing Cassie to demorph from a butterfly after trapping herself as a caterpillar. Karen's father makes a donation to Cassie's father, allowing him to keep the Wildlife Rehabilitation Clinic afloat. Aftran keeps her word and abandons Karen to return to being a blind, helpless slug in the Yeerk pool instead of selling out the Animorphs to Visser Three. However, this book does shake up the status quo a little by revealing that Yeerks are not truly Always Chaotic Evil and directly sets the stage for the Yeerk Peace Movement that will have an important role future books.
- You Killed My Father: The Yeerk in the Hork-Bajir that Cassie killed was Aftran's brother, adding to her angst.
