- Narrator: Marco
Marco learns that his old friend Erek is actually a robot.
Tropes:
- And I Must Scream: The Yeerk that was supposed to infest Erek instead found itself locked in a machine with its own Kandrona generator, meaning it doesn't die after three days away from the Pool. Erek comments that it sees and knows nothing about its situation, meaning that from its point of view, its been imprisoned and it doesn't know why or by who. Is it conscious? He doesn't say.
- Bear Hug: Mr. King defeats Rachel's grizzly bear by hugging her with his android strength, not actually hurting her but not allowing her to move either.
- Bed Trick: Touched upon again when Marco's father Peter tells Marco about how much more agreeable Eva was in the year before her disappearance, sighing about how it was like they'd come to perfect peace, perfect love. Marco concludes that the Yeerk in her just didn't care about all the things Eva did and went along with anything Peter wanted so it could focus on its real goals. In Animorphs: Visser we see that that Yeerk really likes playing happy families.
- Been There, Shaped History: Erek and the rest of the Chee have been hiding among humans for millennia. They've occasionally been close to famous people, and given them advice that was involved somehow in their famous accomplishments, usually just phrases they took to.
- Curb-Stomp Battle: Once Erek is able to rewrite his non-violent Restraining Bolt programming via the Pemalite crystal, he becomes a literal killing machine that effortlessly takes down twenty-four Controllers (including twelve Hork-Bajir).
- Diabolus ex Nihilo: The Howlers were this from the point of view of the Pemalites and the Chee. Erek comments that as far as he knows, the Howlers only wanted to destroy.
- Didn't Think This Through: The Animorphs are able to reach the Pemalite crystal in bat morph, but then realize they have no way to sneak the crystal out. They're forced to assume battle morphs and tear their way out of the building, which leaves them facing down a dozen human-Controllers with guns, and a dozen more Hork-Bajir Controllers right after.
- Dying Dream: After getting eaten, Marco sees a memory of his mother picking him up. It allows him to focus enough to demorph.
- Eaten Alive: Marco gets eaten by a crow while in spider morph. He demorphs in the crow's throat, causing it to explode from within.
- Forgotten Phlebotinum: The Chee are extremely powerful, hardy robots that can project hard light holograms and have been living in human society for millennia. Most don't want to interfere with the conquest of Earth as they feel that would violate the principle of nonviolence, but they don't seem ready to physically stop Erek's small radical faction who feel differently. Despite their nonviolent programming, in this book Erek's able to keep a sentient being helplessly imprisoned indefinitely and the Chee playing his father is able to grab and restrain Rachel in her grizzly bear morph. This suggests that even if they don't fight, can't directly enable violence, and can't just walk into the facility to take the crystal themselves, they could do a lot more for the Animorphs than ends up being the case. Bringing them information to act on and later, impersonating the human Animorphs so they won't be missed on longer missions, and various one-off cases are certainly useful as help but quite limited.
- Foul Cafeteria Food: Marco calls the cafeteria food "The Goo of the Day". Jake comments that said Goo is blue, and food should not be blue.
- Hostage MacGuffin: Marco is able to call off the human-Controllers with machine guns by holding up the Pemalite crystal, which could be destroyed by gunfire. The Yeerks are forced to resort to having their Hork-Bajir Controllers try to pry the crystal from the Animorphs instead.
- MacGuffin: The Pemalite crystal, which the Yeerks are planning to use to create a master computer that can control all computers on Earth. It can also reprogram the Chee, which is Erek's motivation for wanting it.
- My God, What Have I Done?: Erek rewrites his programming, slaughters over a dozen human and Hork-Bajir Controllers each, and is so aghast at what he'd done that he changes his programming right back and allows the crystal to be lost at a beach.
- Mysterious Animal Senses: Marco and Jake in dog morph find out that something's weird about Erek when they realize he has no smell of his own. Later on, Marco and Ax in spider morph are able to see past his hologram with their spider eyes, which operate on a different part of the visible light spectrum.
- Near-Villain Victory: The Animorphs are nearly killed by fighting twenty Hork-Bajir Controllers at once, only surviving because Marco gave the Pemalite crystal to Erek just in time.
- O.O.C. Is Serious Business: Rachel, of all people, breaks down sobbing as she talks about Erek's Curb-Stomp Battle of the Controllers that were menacing the Animorphs. Marco remarks that he didn't really grasp just how horrific that battle was until he saw her crying.
- Perfect Pacifist People: The Pemalites were this. They had an advanced, eco-friendly society that knew nothing of violence.
- Preferable Impersonator: Marco's father sighs happily remembering how his wife Eva changed abruptly. They'd fought and argued now and again, but when this stopped he felt they'd come to a point of perfect love and peace. It was the happiest time of his life. Marco, knowing that that lines up with when Eva was infested and the Yeerk took over, is sickened. Normally, Controllers play normal so well that those around them never notice a change; Peter noticed, and liked it, while his actual wife was in a position of utmost helplessness and misery.
- Shapeshifter Baggage: Ax reveals that the mass lost while morphing into an animal smaller than the morpher is actually temporarily sent to Z-space. The inverse is that excess matter floating in Z-space is used to add mass when morphing into a creature larger than the morpher. It's harmless— unless a ship traveling through z-space should hit a morpher's mass. The odds of that happening are a million to one, but not zero.
- Take Our Word for It: Marco was unconscious when it happened, but Erek's massacre of the Controllers was brutal enough to make Rachel cry upon remembering it.
- Too Good for This Sinful Earth: The Pemalites and their kind, pacifistic society were slaughtered to nothing by the Howlers, leaving only a few hundred Chee behind.
- Violence Is Disturbing: Marco sums up a thesis of the series in his narration.Win or lose, right or wrong, the memory of violence sits inside your head. It sits there, like some lump you can’t quite swallow. It sits there, a black hole that darkens hope, and eats away at everyday happiness like a cancer. It’s the shadow you take into your own heart and try to live with.
- What Measure Is a Mook?: Erek keeps a Yeerk imprisoned in a tiny cell in his android head, wrapped in wires and fed Kandrona rays he generates himself, knowing and seeing nothing, its memories tapped. Marco is momentarily sickened; he hates Yeerks, but this seems much crueler than killing it. It's the most sinister aspect of the Chee - that they can keep a person in sensory deprivation and solitary confinement long term without violating their programming - and no one ever refers to it again, although the personhood of Yeerks becomes increasingly clear over time.
- Worrying for the Wrong Reason: After being told their extra mass when morphing smaller creatures is sent the hyperspace that ships travel through, Marco asks if some ship could splatter the mass across its hull. Ax assures him that's absurd because the ship's shields would hit the mass and disintegrate it instead.
- You Dirty Rat!: The Animorphs get menaced by a rat while morphed into spiders and cockroaches in a vent.
