Animorphs is a science fiction book series by Katherine Alice Applegate. It follows the adventures of five teenagers who gain the ability from a dying alien to transform into any animal they touch for up to two hours. They fight underground against an invasion of parasitic aliens known as the Yeerks, who turn their victims into mindless slaves called Controllers. On the other side are the Andalites, an ancient, wise, and highly developed species that resemble a mix between a blue deer with a human upper body, a second pair of eyes on stalks, and a scorpion tail, similar to centaurs. As explained throughout the story, the Yeerks were only able to leave their home planet and expand to enslave entire planets with the help of the Andalites.