From the back of The Underground:
What's tasty, good for you, and only takes about sixty seconds to make? Oatmeal. And it's making the Yeerks more than a little crazy. Now Rachel, the other Animorphs, and Ax have a new weapon against the Yeerks. Sounds good, right?
Wrong. Because it means another trip to the Yeerk pool. And the possibility of running into Visser Three. And of not being able to get back up to the surface. The Animorphs and Ax have taken chances before and been very lucky. But this time their luck may be about to run out.
A new Planet Hollywood is opening up in the Animorphs' home town, so naturally they want to go. Once Jake is convinced, everyone morphs their personal bird of prey and start celebrity watching. Then Rachel notices a man in a high rise building smash out a window. He's going to jump! Rachel and the others manage to catch him by snatching him from midair once he takes the big leap. The Animorphs barely save him in time. Later, from reading the newspaper, they find out the man has been institutionalized. His family thinks he's crazy, since he's been speaking about aliens living in his head. This, of course, tips the Animorphs off. What is he talking about?
They decide to pay him a little visit in the looney bine (I'm so politically correct) to find out what the man was talking about. They are afraid he really knows about the Yeerks and must know the truth. Rachel, Jake, and Marco manage to slip into the heavly fortified building by morphing roaches and climbing in a crate of food. This food turns out to be bananas, which Rachel finds out first hand can house tarantulas.
Once inside Rachel demorphs in the crazy man's, George Edelman's, bathroom. He comes in and Rachel tells him she's an Andalite. Edelman explains that he has a insane Yeerk living in his head. The Yeerk went insane from having Edelman ingest instant maple and ginger oatmeal, which freed the Yeerk of the need for Kandrona rays. The crazy Yeerk would live in Edelman's head until the day he died because of its new Kandrona-free condition. Edelman was sad, knowing he'd live the rest of his days in a mental institution because of something he can't control.
The Animorphs decide to go down to the Yeerk Pool to dump some oatmeal into the food supply at the Yeerks' cafeteria. Not just any oatmeal, instant maple or ginger oatmeal specifically. But when they tried to follow a Controller down, they were detected and almost killed by a Bio-Filter, which kills anything it is not programmed to accept, the Controller presently in the room. The Animorphs are now locked out of the Yeerk Pool, at least, until Cassie has a bright idea.
Cassie's idea is simple, morph moles and dig a tunnel down to the Pool. This takes much longer than anticipated, however, and they don't come out in the Pool area, but into a cave. The next day they come back and all go down to this cave, which they discover is a bat cave. The bat cave does lead to the Yeerk pool, however, so now they have their own entrance. One problem, they can't reach the mole tunnel they used to enter, so they've got to go to the Pool and out of one of those entrance.
When they reach the Pool they run into some problems. Flying robot guards that shoot Dracon beams begin to fly at them, and one severes Rachel's wing completely off. She falls into the Yeerk Pool. At first Rachel is worried someone will see her and be suspiscious, but nothing happens. Yeerks even bump up against her and don't notice her. Rachel manages to flounder over to the loading deck and get under it. She demorphs, then goes to ant morph. Rachel then climbs up on the pier, snags onto a Controller and is out of there.
Rachel demorphs and happens upon a Dracon beam. She hit a Hork-Bajir and a human, whose shoes, jacket, and glasses she takes. Then she comes out of her little shack. Rachel is just in time to see Ax being captured. He too was hit by the robot's Dracon beams and had to demorph. Jake and Tobias also had been captured. Rachel then overheard a conversation about confiscation of the "drugs" and where they were guarded. Now she knew where some oatmeal was, ready to be given to hungry Yeerks.
Rachel finds Cassie, who has demorphed and looking for her. They meet just in time to see Visser Three land and start throwing orders around. Everyone is to be checked. Rachel and Cassie know they'll be found so they head to the cafeteria to get away and happen upon a gorilla, Marco. Rachel and Cassie morph and bust right through the wall to get the oatmeal, which they dump in the Yeerk Pool and hit with the Dracon Beam. This creates enough pandamonium that Jake, Tobias, and Ax escape and they all get away.