The Animorphs save a man named George Edelman from jumping out of a building. They later hear that he has been committed to an asylum for talking about aliens living in his head. They bust in to talk to him, and he reveals that some Yeerks have discovered a way to survive without the Kandrona: eating Instant Maple and Ginger Oatmeal. However, the oatmeal is extremely addictive to the Yeerk, who goes insane and never leaves the host body. The Animorphs decide to use this information to hurt the Yeerks by dumping oatmeal into the Yeerk Pool.

They attempt to morph flies and follow a controller into the pool, but find that the Yeerks have installed "Gleet Bio-Filters" on all Yeerk Pool entrances which destroy any unauthorized life forms that attempt to enter. They decide instead to morph moles and dig their way in. On the way through their tunnel, they find a bat cave. They morph bats and fly into the Yeerk Pool, but are attacked by hunter robots. Ax, Jake and Tobias are captured, and Rachel falls into the pool. She morphs an ant, gets out, steals a Dracon beam, and disguises herself as a human-Controller. After overhearing that there is a stash of the oatmeal near the pool confiscated from addicted human-controllers, she finds Cassie and Marco just as Visser Three arrives to receive the captured "Andalite bandits." Rachel throws a barrel full of oatmeal into the pool and threatens to burst it open with the Dracon beam unless the others are released. Visser Three is about to allow the Yeerks in the pool to be sacrificed in order to capture the "Andalite Bandits", so Rachel tosses him in as well. They begin to escape with the others, but as they are escaping the Visser begins morphing, so Marco shoots open the barrels of oatmeal. Rachel uses the Dracon beam to collapse the tunnel on them, and they dig to the surface as moles.

The book ends as Rachel's mom reports that George Edelman escaped from the asylum with the help of a psychic grizzly bear.

Jake -Peregrine falcon, seagull, cockroach, housefly, mole, brown bat .

Rachel -Bald eagle, bottlenose dolphin, seagull, cockroach, housefly, mole, brown bat, black garden ant, African elephant, grizzly bear .

Tobias -Human (himself), housefly, mole, brown bat .

Cassie -Osprey, housefly, mole, brown bat, wolf .

Marco -Osprey, seagull, cockroach, housefly, mole, brown bat, silverback gorilla.

Ax -Northern harrier, human (main), housefly, mole, brown bat .

Visser Three -"Pterodactyl creature"