- Narrator: Ax
A group of Andalites arrive on Earth with the intent to free humans from the Yeerks.
Tropes:
- Always Someone Better: Estrid is an estreen on a higher level than Cassie, as her morphing is fast, completely controlled, and she can morph a full set of clothing. Ax describes her morphing to human as her Andalite body shrinking into a fleshy ball, which then deflates into a human girl.
- Blatant Lies: Estrid wonders why Ax cares so much about humans, thinking it's the cinnamon buns. Ax remembers the hosts freed from their cages who used themselves as Human Shields against the Yeerks, then sardonically says yes, it's the cinnamon buns.
- Big Damn Heroes:
- Unit 0 arrives and unintentionally bail the Animorphs out of a bind. They were only looking for Ax, the rest were incidental.
- Tobias and Gonrod use Unit 0's ship to blow a hole through McDonald's and into the Yeerk pool, rescuing the other Animorphs and Estrid.
- Boom, Headshot!: Aloth dies to a headshot from Arbat.
- Boxed Crook: Gonrod was court-martialed for cowardice while under fire, while Aloth was imprisoned for selling the organs of fallen soldiers on the Andalite black market. Both are used on Arbat's mission as expendable muscle.
- Breaking the Fellowship: Subverted. The Animorphs apparently disband when Unit 0 reveals they're only here to kill Visser Three. Ax and Estrid spy on them as they fall to arguments and infighting despite Cassie's best efforts. These turn out to be staged and/or played out by Erek's holograms, which Ax only reveals late into the book.
- Broken Pedestal: Estrid thought of Arbat as her beloved professor who believed in her despite their society saying he shouldn't. Ax reveals to her that he was sending her to her death, and Arbat claims it was to protect her from criminal charges. Estrid doesn't blink much when she joins Ax in leaving him for dead.
- Cain and Abel: Arbat is Alloran's brother, tasked with killing him to kill Visser Three. He hesitates when the moment becomes available because the Visser can use Alloran's voice to torment him.
- Cold Open: The book opens with Mr. King caught by two human-Controllers and the Animorphs trying to rescue him.
- Cold Sniper: Aloth can't morph, so his use in the mission is to be the ranged fighter. He's cynical and brutish while on duty, but he is friendly to Ax, though still cynical.
- Decoy Leader: Gonrod is made out to be the commander of Unit 0's mission. He's actually an Unwitting Pawn for Arbat.
- Dirty Coward: Gonrod is too cowardly to actually try to fight Yeerks. It's why Arbat chose him for the mission, as he could take command during a critical point without Estrid or Aloth questioning him.
- Eaten Alive: Arbat is last seen cornered by Taxxons. It's obvious that the Yeerks didn't manage to take him alive.
- Left for Dead: Arbat is left on the Yeerk pool pier by Ax and Gonrod. He's in human morph, injured by Estrid, and Taxxons are nearby. The Animorphs don't need confirmation that he died.
- Legally Dead: With Mr. King's help, Ax finds that Arbat, Gonrod, and Aloth are all listed as killed in action, and Estrid has no listing within the academy, giving away that Unit 0 is on a suicide mission.
- Love Hurts: Estrid is unable to completely hide how much Ax's rejection stings when they part ways, while Ax is comforted by Cassie much later where no one can see.
- Mexican Standoff: The first meeting between the Animorphs and Unit 0 turns into one, with Arbat and Aloth with their tails and shredders at the ready, while Marco and Cassie sneak up on them in cobra and viper morph, respectively, while Jake and Gonrod try to out-leader each other. Jake wins.
- The Mole: Ax joins his fellow Andalites not out of loyalty to his species, but to make sure they won't cause trouble for Earth out of loyalty to the Animorphs.
- Nepotism: Aloth privately accuses Estrid of being some high ranking officer's niece as an explanation for how a female aristh could be included on a sensitive mission. Estrid later explains that she's no one's niece, but she is Arbat's student.
- Out-of-Character Alert: Ax singles out Estrid spying on the Animorphs because her rabbit morph follows him to Cassie's barn, which isn't something rabbits do.
- O.O.C. Is Serious Business: Ax coolly watches Cassie storm off from an argument with Jake and Marco while Rachel tears apart the mall as a grizzly bear, all of which are highly out of character for the Animorphs. Sure enough, they're merely staging to Estrid that they won't be getting in the way of Unit 0's mission.
- Scaled Up: Cassie acquires a pit viper to aid Marco's cobra in threatening Unit 0 into complying with Jake.
- Suicide Mission: Estrid, Gonrod, and Aloth were not expected to survive Unit 0's mission, while Arbat was meant to be the designated survivor. Thanks to the Animorphs, Gonrod and Estrid survive.
- Sweet Tooth: Estrid loses control of her human morph when she comes across a candy store. Ax and Tobias find her crouched in front of the jellybean bin, almost crying with joy and eating entire handfuls.Estrid: Beanzuh! Jelly beanzuh! Beanzuh! Zuh! Zuh!
- Synthetic Plague: The Quantum Virus that Estrid designs to kill the Yeerks.
- Temporary Love Interest: Ax quickly starts crushing on Estrid and vice-versa, and it's not hard to see why. From Ax's point of view, Estrid is beautiful, a genius, a talented morpher, subversive among Andalite society's sexism, and the first female Andalite of his age that he's seen in a long time. From Estrid's point of view, Ax is a war hero surviving against impossible odds in spite of being held back by an inferior species, knowing more tricks about Yeerk strategies and Earth fauna than any War Prince she's met. Ax's crush on Estrid evaporates when it becomes apparent that she thinks little of anyone outside of the Andalites, and rejects her offer to return to the Andalite homeworld with her.
- Unreliable Narrator: Ax deliberately withholds information from the reader, making it look like he's abandoned the Animorphs to self-destruct while he saves Earth with Estrid. It's not until late in the book when Unit 0 starts fighting among themselves that he calls for his human friends to regain control of the situation, revealing he had been The Mole the whole time.
- Villains Want Mercy: After nearly wiping out humanity through disease and pretending to be a human-Controller to send Hork-Bajir Controllers after the Animorphs and Estrid, Arbat begs them to save him as well. None of them do.
- Well-Intentioned Extremist: Arbat's goal is to use a Quantum Virus on the Yeerks, though the virus has a chance to start killing humans along with them. He's not concerned about that last part.
