- Narrator: Marco
Visser One returns to Earth, setting up an underwater base for her own plans of conquest.
Tropes:
- Comically Missing the Point: A Leeran-Controller realizes that the gorilla standing next to it is a human in morph (Marco), and tries to relay this to Visser One. She assumes he thinks the gorilla is some kind of human, and "corrects" that it's merely a similar creature. Marco knocks it out before it can tell her otherwise.
- Devious Dolphins: Tobias is forced to acquire a dolphin in front of a crowd. To make matters worse, the dolphin fails to go into a trance and proceeds to make a game out of nearly drowning the red-tailed hawk latched onto its back.
- Drama-Preserving Handicap: Thanks to the brain amplifier that gets installed into the Animorphs when they're in shark morph, they're unable to use their insect morphs while they're in the underwater base, which forces them to pretend to be human-Controllers (in Marco's case), be birds (Ax, Marco, and Tobias), hide (Ax and Tobias), or use battle morphs (Jake, Rachel, and Cassie, joined later by the others).
- Enemy Civil War: Visser Three openly attacks Visser One's operation, hoping to discredit her somehow.
- Entertainingly Wrong:
- Ax assumes the underwater tunnel at Ocean World is a human-made hologram. He's stumped to learn that they're actually underwater, and the only thing preventing them from drowning is "badly made human plastic".
- Visser One comes face to face with Marco, and assumes he's one of the four human-Controller technicians she called for, which he's forced to roll with to avoid blowing his cover.
- Exploding Fish Tanks: The Animorphs are cornered in the underwater tunnel by a cop-Controller, who luckily assumes them to be delinquents helping an Andalite. Marco then calls attention to the "badly made human plastic" surrounding them, and Ax gets the hint, slicing the aquarium open with his tail and letting water pressure do the rest.
- Five-Second Foreshadowing: Watching from below in tiger shark morph, Ax warns the others that some fish are approaching their dolphin morphs. Rachel is unimpressed, until Ax comments on said fish’s rectangular heads. The Animorphs then realize they’re about to be set upon by hammerhead sharks.
- Fowl-Mouthed Parrot: To save some parrots from being used for advertising the Rainforest Cafe, Jake, Cassie, Marco, and Rachel morph into said parrots and trash talk the restaurant.
- Internal Reveal: Marco reveals that his mother is Visser One's host to stop Rachel from killing her.
- It Can Think: The hammerheads around Royan Island have been equipped with brain amplifiers, making them capable of ignoring their instincts and set up traps for dolphins and unfamiliar sharks. This was done to make hammerheads fit for Yeerk infestation, as a shark-Controller would be a devastating weapon in Leera.
- Lack of Empathy: When he's unable to compose himself in front of her, Visser One coldly tells Marco that Eva is screaming and crying at the sight of her son apparently being a Controller, but she's in perfect control of her host body, regardless. She implores the (supposed) Yeerk controlling Marco to reassert dominance again.
- No Ontological Inertia: The shark brain implants dissolve once the underwater base is destroyed. This is done to prevent humans from accidentally finding Yeerk technology by fishing up one of the many hammerheads they modified.
- Refuge in Audacity: Marco accidentally comes face to face with Visser One, who assumes he's the technician on loan from Visser Three she was expecting. When asked where the other technicians are, Marco claims Visser Three killed them. She buys it immediately.
- Telepathy: Leerans have the ability to read the thoughts and emotions of any creature nearby. This makes a stealth invasion impossible for the Yeerks, but Leeran-Controllers are especially dangerous to the Animorphs since they can easily find out who they really are.
- Threatening Shark: The hammerhead shark morph is described as being focused exclusively on food. If there is no food, it goes to find more. It doesn't know fear, and doesn't care about pain. It's a predator primarily concerned with its next meal.
