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The story revolves around five humans: Jake, Marco, Cassie, Rachel and Tobias, and one alien, Aximili-Esgarrouth-Isthill (nicknamed Ax), who obtain the ability to transform into any animal they touch. Naming themselves "Animorphs" they use their ability to battle a secret alien infiltration of Earth by a parasitic race of aliens resembling large slugs called Yeerks, that can take any living creatures as a host by entering and merging with their brain through the ear canal. The Animorphs fight as a guerilla force against the Yeerks who are led by Visser Three.