While this isn't the most gripping plot written for the Animorphs there's a great deal of tongue-in-cheek humor in it. The mission is to spy on horses which have been infested by Yeerks (see Animorphs 1: The Invasion). To what end, they can't imagine, but it obviously has something to do with "The Holy Grail of Conspiracy nuts" Zone 91. This spoof of Area 51 is a high security military base which is guarding The Most Secret Thing on Earth. Notice the importance stressed by every first letter being capitalized. Some of the other reviews mention what it is, but I'm not going to spoil the fun if you're about to read the book. It turns out to obviously be an alien artifact, and the Yeerks want it so they can wipe out any and all proof that humans might make a connection that there really is alien life in the universe. The Animorphs go through an interesting process of acquiring the DNA of race horses so they can morph into them and infiltrate the dreaded Zone 91, involving explaining Ax as a blue horse. "Someday all horses will be blue," they lamely explain as they're caught, are forced to scatter, and Cassie ends up on the race track. The plot isn't the strongest in this one, but the book is simply a lot of fun. Cassie gets a bit more development in the fact she simply is not the coolest person under fire, which leads to even more humor. Marco, as always is the witty one with the quick logic to make the insane suddenly wise. For people who may have enjoyed X-Files and similar programs, but don't take the Area 51 conspiracy theory too seriously, this is a great, quick read, full of good laughs.