When the Animorphs go to see a performance of The Lion King at a school assembly, the Ellimist freezes time and appears to enlist their aid. He tells them there is another force as powerful as him, the Crayak. The Ellimist reveals that this creature is the blood-red eye Jake saw when the Yeerk in his head died in the sixth book, The Capture.

The Ellimist tells that when Crayak first appeared, they waged a war that destroyed a tenth of the galaxy. They realized that they must instead have a much more subtle battle. All the times the Ellimist has helped them have all been small moves in this game.

The Crayak is targeting another race, the Iskoort, and this interferes with the Ellimist's concealed agenda. The fate of the Iskoort will be determined through a proxy battle with rules of engagement. The Crayak and the Ellimist will each choose seven combatants to face off on the Iskoort homeworld. Crayak has chosen seven of his shock troops, the Howlers, members of the race that destroyed the Pemalites.

The Animorphs choose Erek King as their seventh, and they are taken to the Iskoort world, a huge metropolis miles above the ground. While the Iskoort turn out to be bizarre and grating, they are not actually evil. The Animorphs and Erek enlist a young Iskoort trader named Guide to show them around.

Eventually, they run into one of the Howlers. Even alone, it proves to be completely superior in battle. They barely survive the fight, while the Howler quickly recovers from its injuries. Regrouping, they buy copies of Howler memories with Guide's assistance. (Ax determines that the Iskoort homeworld is sufficiently isolated from Yeerk influence for this transfer of information to be of negligible risk to Animorph secrecy on Earth.) Erek watches the purchased Howler memories, revealing all the massacres the Howlers have committed. They determine that the Howlers were created by Crayak, and that they have collective memories, giving them each thousands of years of battle experience. There are no memories of the Howlers ever being defeated.

The Animorphs find a new safehouse as Guide mentions a personal errand: A function of Iskoort physiology requires the Iskoort to separate into two halves, the Isk, the body, and a slug-like entity, the Yoort. Guide continues to explain that every three days, Iskoort must separate so the Yoort can feed. Upon recognition of the similarities to the Yeerks, chaos ensues, interrupting Guide, and violence is narrowly averted. Guide explains early Iskoort history, closely resembling the Yeerks' current existence of conquering and enslaving alien races. The Iskoort ancestors disliked their evolved biology of parasitism, instead employing genetic engineering to create a primary host, the Isk, and additionally altered Yoort physiology to become true symbiotes in order to maintain the established standard of living but removing the necessary violence. At this point, the relevance of the Iskoort to both the Ellimist and the Crayak is revealed. Should the Yeerks ever interact with the Iskoort, the Yeerks would learn of a possibility for nonviolent existence. Since the Yeerks are conquerors, the desired nonviolence of some Yeerks and dissidence within the Yeerk Empire would delay Crayak's goal of dictatorship over the galaxy and ultimately, the universe. Even worse (in Crayak's view), the Yeerks could abandon their current imperialistic nature, forcing Crayak to find new conquerors to unite the galaxy under his rule.

After several narrow escapes, Jake and a Howler both fall off a building towards the ground below. Jake acquires the Howler and quickly morphs to peregrine falcon, pulling to safety with a spiteful quip as the Howler plummets to its death. Back atop, Jake reunites with the others. Jake and Cassie run to each other and kiss, prompting Rachel to say that "it's about time".

Jake morphs the Howler under guard with the others under strict orders to kill him if he loses control to the instincts of a killer created by Crayak. He finds no such things. Instead, he discovers that Howlers have the personalities of little children: the Howlers consider their battles to be fun and exciting games and have no perception that they are hurting real people and annihilating entire civilizations.

The Animorphs make the memory-recordings, along with Erek and Guide. When the Howlers next attack, the Animorphs use Jake in Howler morph to stun one of them and force the entire combined memories of the eight of them (the Animorphs plus Erek and Guide) into the Howlers, along with the knowledge that the non-Howlers are real beings. Crayak is forced to destroy all six Howlers before the memories can contaminate the collective memory. The Animorphs win the standoff on behalf of the Ellimist.

The Ellimist transports the Animorphs to n-dimensional space in order to talk with Crayak face-to-face in a way that the Animorphs can see. Crayak concedes defeat and agrees that the Iskoort will live. Jake morphs the Howler in order to determine the effectiveness of the Animorphs' "memory contamination" attack. He finds nothing in the collective Howler memory relating to the Iskroot homeworld. Since all Howlers share a single, collective memory and Jake in Howler morph has no memory of the fallen Howler falling by Jake's hand, Jake determines that Crayak obliterates unsuccessful Howlers before these memories of defeat blemish the collective memory, ensuring that only a streak of flawless Howler victories remain; however, a single incident from the Iskoort homeworld has penetrated the collective memory: that of Jake and Cassie making out at night with the Howler D.N.A. still in his body.

Taking his warriors back to Earth, the Ellimist talks with the Animorphs further. He reveals that in a few months' time, on the next Howler raid, the Howlers will attempt to kiss their targets. The contaminated Howlers are no longer useful as Crayak's shock troops. The Ellimist also reveals that in three hundred years' time, the Iskoort will meet the Yeerks.

The following night, Jake tries and fails to turn his dreams from the Howler he sent to its doom.

Jake -Siberian tiger, housefly, peregrine falcon, Howler.

Rachel -Grizzly bear, housefly, bald eagle .

Tobias -Housefly.

Cassie -Wolf, housefly, osprey.

Marco -Silverback gorilla, housefly, osprey.

Ax -Housefly, northern harrier.