- Narrator: Jake
During the launch of Visser Three's new Sea Blade, the Animorphs and the Yeerks run afoul of a hostile underwater race.
Tropes:
- Big Damn Heroes: Tobias shows up to rescue the team just in time before their vivisection by ambushing the surgeons while in Nartec morph.
- Bio-Augmentation: Visser Three tried experimenting on some enslaved Hork-Bajir to make them aquatic for the purposes of sending them to find the Pemalite ship. The Hork-Bajir just painfully mutate and die.
- But I Read a Book About It: Marco and Jake are normally Book Dumb, but thanks to The Hunt for Red October, they know the difference between active and passive sonar.
- Dead Guy on Display: The Nartec make a habit of making an exhibit of every ship that falls into their hands, ending by taxidermizing the crews of those ships.
- Enemy Mine: The Animorphs are once again forced to work with Visser Three to fend off a hostile third faction. The alliance ends when the Animorphs allow the Sea Blade to be torn apart by torpedoes while the Visser's morph is unable to fight them in the water.
- Fish People: The Nartec are a race of amphibious humanoids with gills, blue skin and webbed hands who inhabit a city beneath the ocean. They're not aliens, but humans who have been severely mutated over generations. They're amphibians, but they've degenerated so much that few can actually live on land for long. They kill normal humans to get new DNA and keep their species going.
- Human Subspecies: The Nartec are descended from humans who lived in a city that sank beneath the waves, and over time adapted to their new home — a process sped along by the fact that the glowing rocks they use to light up their city are highly radioactive. Unfortunately, the same radioactivity has cursed them with a host of genetic illnesses and resulting short lifespans, resulting in most Nartec being horribly frail or dying young, and the Nartec as a whole being headed for extinction — something they try to prevent by harvesting fresh DNA from baseline humans, with whom they're still compatible.
- It Can Think: Jake is shocked by how intelligent the orca morph is. In fact, the orca's instincts are aware that it's sharing a headspace with a human mind.
- Just in Time: Tobias rescues the rest of the Animorphs just as they get sedated for vivisection. His excuse is that the Nartec morph he acquired is way too sluggish outside of the water.
- Killing for a Tissue Sample: The Nartec want to kill the Animorphs with an elaborate machine to get their DNA to increase their gene pool. They aren't interested when it's pointed out that there's easier ways to get a DNA sample.
- Left Hanging: The Nartec are never brought up again after this book, even though Rachel promises to bury them as soon as the Yeerks are no longer a problem.
- O.O.C. Is Serious Business: Cassie is upset enough by the deaths of the mutated Hork-Bajir to act more bloodthirsty than usual while hunting the Sea Blade.
- Omniglot: Queen Soco knows how to speak at least every European language fluently.
- Outside-Context Problem: Neither the Animorphs or the Yeerks ever considered that there would be a hostile race of mutant aquatic humans under the sea.
- Quality vs. Quantity: The Nartec are pathetic fighters on land, but there's enough of them to overwhelm even Visser Three in morph.
- Skewed Priorities: Ax is more offended that the Nartec considered his Andalite DNA unsuitable for their purposes than the fact that they were about to vivisect his friends.
- Strapped to an Operating Table: How the Nartec vivisect people. The sedative they use only stops the victim from moving, and does nothing about pain or consciousness. Jake, Cassie, Rachel, and Marco all end up this way before being rescued by Tobias.
- Sub Story: The first third of the book is the Animorphs chasing down and destroying the Sea Blade with their orca and sperm whale morphs.
- Taxidermy Is Creepy: The Nartec taxidermize the crews of the ships they capture after they're finished vivisecting them. This ends up being the fate of the Hork-Bajir Controllers who were manning the Sea Blade.
- This Is Unforgivable!: Jake notices that Cassie is more gung-ho about their mission than usual, concluding that she's determined to avenge the Hork-Bajir that were experimented on by Visser Three.
- Underwater City: The Animorphs, plus Visser Three, get trapped in a city in an underwater cave. The inhabitants are amphibious mutants whose race is dying, which means that they harvest any humans who sink for DNA.
- Weaksauce Weakness: Visser Three's Luminar morph can vent fire and electricity and kill Nartec by the dozens, but submerging it in water renders it helpless.
