Animorphs is a  a book series. It is about  a group of kids who meet an alien from a race called Andalites at a construction site. The Andalite tells them about another species of aliens called Yeerks, who are invading Earth.These Yeerks are slugs who are weak in there natural state but have the ability to go inside a persons brain and take control of that person,know all his thoughts, and thus have the ability to act like the  n they have infested, which makes it hard to know if a person is infested by a Yeerk. The Yeerks have infested other races of aliens, namely the Hork-Bajir and the Taxxons.One Yeerk is leading the invasion,his name is Visser Three ,and is the only Yeerk who has infested an Andalite.

The group of  kids I mentioned earlier were given the power to change into any animal they wish, but first they have to touch the animal the want to morph. If they stay in that form for more than two hours, they will be in that morph permanantly

. These kids use this power to fight the Yeerks.