is a science fiction book series by K.A. Applegate about a group of kids who are given the power to morph any animal that they want. However, this morphing power is given with a price. The alien who gave them the powers, an Andalite named Elfangor, gave them the powers for a reason: to stop the invasion of the Yeerks. The Yeerks are a parasitic race of sluglike aliens who infest a species by crawling in their brain and thereby controlling the creature. (Infested humans are called Human-Controllers.) The Yeerks' main goal is the complete control of the planet earth and her resources.
The Animorphs-- Jake, Cassie, Rachel, Tobias, Marco, and Ax, Elfangor's brother, must help to fight the invasion, while at the same time eluding Human Controllers and various other aliens that are in alliance with the Yeerks.
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to read an excerpt from Book 1, "The Invasion."