- Narrator: Cassie, Aldrea
The last of the Arn comes to Earth to ask the Animorphs and Toby for help.
Tropes:
- And Now for Someone Completely Different: Aldrea takes over half the narration of the book after she's implanted into Cassie.
- Bait-and-Switch: Quafijinivon says that Aldrea's Ixcila will gravitate to the person most similar to her. Based on his description of her, the Animorphs assume she'll go to Rachel or Toby, but she goes to Cassie instead.
- Category Traitor: Ax doesn't say it out loud, but he's incredulous at the fact that Aldrea considers herself a Hork-Bajir rather than an Andalite.
- Clone Army: Quafijinivon harvests the DNA of the free Hork-Bajir to make his own army of Hork-Bajir guerillas back on their homeworld.
- Fake Defector: Ax, Aldrea, and Cassie conspire to make it look like Aldrea is refusing to leave Cassie to get Toby to return to Earth as a "hostage" instead of remaining as a lackey to Quafijinivon.
- Fish out of Temporal Water: Aldrea was recorded before her death, so she's shocked to find that a few decades have past and she's on a different planet.
- Ghost Memory: The Arn have the ability to record the memories and personality of beings and store them for later use. Aldrea and Dak were recorded, though Dak's was destroyed, and in the present day, Aldrea is summoned to find supplies she left behind before she died.
- Hypocritical Humor: Despite Aldrea constantly referring to herself as a Hork-Bajir, she engages in the same Cultural Posturing about Andalites at the same time that Ax does.
- Last of His Kind: Quafijinivon claims to be the last of the Arn, and he knows he's going to die soon.
- Protection in Mouth: The Animorph's plan to assault the Yeerk pool. Cassie morphs into an osprey and carrying the others in flea morph in her mouth, then rapidly demorphs mid-air and re-morphs into a humpback whale before landing in the pool. Her whale morph's giant mouth acts as a shelter for the other Animorphs while they morph into their battle morphs.
- Shapeshifter Mashup: Cassie is able to use an advanced morphing technique to start morphing to whale while not completely demorphed from osprey. She does this to keep the wings for as long as possible, as she's falling from the sky and into a Yeerk pool while doing so.
- Sharing a Body: Cassie spends most of the book sharing her head with a memory recording of Aldrea.
- Skewed Priorities: Marco wants his Chee double to ask out a girl at his school. Erek sarcastically says sure, the Chee that impersonates Marco will take time off from his job to help Marco's love life.
- Teeth-Clenched Teamwork: Aldrea isn't happy to have to work with immature human teenagers, a sketchy Arn, and an Andalite who openly disdains her. They all eventually develop respect for each other, with the exception of Quafijinivon.
- Your Days Are Numbered: Quafijinivon knows the exact time and date of his death, which is why he wants his home planet freed from the Yeerks sooner rather than later.
