They began to say that somewhere in the Arid Lands there is a secret Area-91 - a US Air Force military base where they are studying the wreckage of an alien ship that landed in the fifties. It turned out that the Yeerks are showing increased interest in Area-91. Therefore, the Animorph friends could not ignore this fact and went there to investigate. The Yeerks, as specialists in space objects, expected to receive valuable information about alien technology. But it turned out that the alien ship was not just a ship... However, only Ax was able to guess, and even then not right away, what it was that was actually discovered in the Arid Lands many years ago.