Cassie and Tobias think there is another Andalite on Earth because they can hear noises from a distance. They went into convulsions, and they heard some strange thought-speak message. It sounded like something had crash-landed and needed help. Rachel was at her father's, so the Animorphs checked it out and found out that there is another Andalite on Earth. The Andalite has contacted Visser Three as well.
Now it's a race to find the source. If the Yeerks find the messenger first, the Andalite will become a Controller. Cassie, Marco, and Jake find the Andalite, but The Andalite does not believe them at first when they say that they are allies of the Andalites. Cassie morphed into a Skunk and saved Ax, and they escaped by morphing into Butterflies. It is revealed that Ax acquired all the DNA Patterns of each Animorph (except Tobias) and was made into a new person.