- Narrator: Cassie
Cassie and Tobias receive a message in their dreams, which appears to be coming from the ocean.
Tropes:
- Big Damn Heroes: A pod of humpback whales, including the one the Animorphs saved from sharks earlier in the book, arrives to ward off Visser Three before he can eat the "Andalite bandits".
- Big Eater: The seagull morphs are constantly distracted by garbage which may contain food.
- Didn't Think This Through:
- Cassie morphs to squirrel to catch a fox that's been eating her patients. The squirrel brain overtakes her instantly, putting her in danger from not only the fox, but Tobias as well.
- Ax's message was meant to reach another Andalite. An Andalite did receive it, but said Andalite is Visser Three's host, putting Ax in the Yeerk's crosshairs.
- Extinct in the Future: A potential future variation appears. Ax reveals that all species of life on Earth will be wiped out by the Yeerks, except for those required to feed their human hosts, if they succeed in taking over Earth.
- Fantastic Flora: The trees in the Andalite dome are described as broccoli and asparagus shaped, some are blue and orange, and the water contains green crystal formations.
- Gaia's Vengeance: When Visser Three is chasing the heroes, Cassie calls out to the whale she'd been communicating with earlier, and two humpbacks and several sperm whales respond to fight him. Her narrations says that it seems like the sea itself is fighting back.
- Good Cannot Comprehend Evil: Cassie refuses to believe Ax when he explains that the Yeerks will destroy every part of Earth's natural ecosystem that can't be used to feed humans.
- Half the Man He Used to Be: Marco is bitten in half by a shark while in dolphin morph, forcing him to demorph in the open sea.
- Humanity Ensues: Ax morphs to human for the first time. He stumbles around because he can't figure out how to walk with two legs and no tail, plays with the human ability to make noise, and constantly turns his head because he no longer has stalk eyes to look around with.
- Playful Dolphin: The dolphin morph's instincts are to frolic around and have as much fun as possible. The only exception is when they encounter a predator.
- Sapient Cetaceans: The dolphin morphs are noted to be intelligent, so much that Cassie considers morphing into a dolphin similar to being a Yeerk in a host body. The whales they encounter are able to speak with the Animorphs while they're in dolphin form.
- Shapeshifter Mashup: Ax demonstrates the Frolis maneuver to acquire a human morph from the DNA of Cassie, Marco, Jake, and Rachel. The end result is an androgynous, Ambiguously Brown human boy.
- Sixth Ranger: Ax joins the team in this book, bumping their numbers up to six.
