- Narrator: Jake, Isaiah Fitzhenry
A free Hork-Bajir gets captured by the Yeerks, so the Animorphs must defend the free Hork-Bajir from an invasion.
Tropes:
- And Now for Someone Completely Different: Jake finds a diary chronicling the war effort of an American Civil War soldier named Isaiah Fitzhenry. Half the chapters of this book are devoted to Isaiah's point of view as he fights his war.
- Big Dam Plot: Jake sees some beavers building a dam, and gets the idea to use beaver morphs to set up a trap for the invading Yeerks. When they arrive, the Animorphs break the dam and wash them all away.
- Breaking Old Trends: In a first for him, Visser One reuses the eight-headed fire-breathing giant morph he used in The Invasion instead of breaking out a new alien monster morph.
- Expospeak Gag: Ax refers to building a beaver dam as "constructing an organic cellulose hydrological attack assemblage".
- Giant Wall of Watery Doom: Once Ax breaks the dam, this is what comes crashing through the valley, crushing everything unfortunate enough to still be in it. Jake in tiger morph and Visser One in fire hydra morph are both swept away by the current, but the tiger has the advantage in the water.
- Headbutting Heroes:
- The Animorphs have spent time arguing about whether to make the invasion public or not. An open invasion is still a death sentence for them, but arguments in favor cause the team to fight amongst themselves.
- Jake would rather evacuate the free Hork-Bajir from the valley, but Toby would rather stand and fight than lose her people's home.
- The Hero Dies: Isaiah is gunned down by the Confederates in his last chapter, and only manages to write a few more lines in his diary before dying.
- Hope Is Scary: The Animorphs now feel this way about the arrival of the Andalite fleet, mostly because none of them, not even Ax, believe the Andalite will come to help them anymore.
- Humans Are Bastards: The Animorphs and Toby discuss this as they consider the possibility of the free Hork-Bajir being discovered by normal humans. Marco points out that humans won't tolerate another human who roots for a different football team, and Ax claims that humans are one of the most intolerant species in the galaxy.
- Killed Mid-Sentence: The last lines in Isaiah's diary are "I fear I am killed. I hope I have done my best. I hope—" Jake hopes the same thing, wondering how his story will end.
- Logical Weakness: Visser One's fire hydra morph is as weak to water as one can expect, especially a crashing wave from a broken dam.
- Nothing Is the Same Anymore: The free Hork-Bajir are forced to relocate, knowing that the Yeerks will just come back with superior firepower.
- Plot Parallel: Isaiah Fitzhenry's war mirrors Jake's. Both of them are teenage boys in charge of fighting a powerful enemy determined to make the losing faction into slaves, and little hope that backup will arrive for them. They're also told to learn a lesson about trusting previously enslaved people to help them fight the enemy. Each time the story flashes back to Isaiah, something about his adventure is mirrored by something in Jake's.
- Pyrrhic Victory: Some of the free Hork-Bajir and some of the human campers die in the flood used to kill the invading Controllers. The survivors are left to trudge away in shellshock before the Yeerks have time to regroup.
- A Storm Is Coming: Jake's half of the book contains a foreboding feeling, enough to emphasize that the Yeerks will begin the open invasion soon. Chapter 17 ends with bold text that reads The countdown has begun. It will all be over very, very soon.
- Straw Civilian: The Animorphs approach a group of human campers who happen to be in between the Yeerks and the free Hork-Bajir valley to get them to evacuate. Jake is forced to morph to Hork-Bajir to get them to listen, and they respond by treating the plot like a episode of Star Trek: The Original Series. Most of them don't even realize that they're risking death by staying until the Yeerks are imminent.
- Workaholic: The beaver morph is described as one. Jake notices that the beaver's primary thought is working on the dam, and it's itching to get it done.
