by K.A. Applegate

The Animorphs series is the series about the early adolescents Jake, Marco, Rachel, Cassie, and Tobias, who are fighting against the aliens called "Yeerk", a parasitic snail-like creature. Yeerks live inside human body, in the brain to exact, and take over the central of control system for the whole body. Thus, a human with a Yeerk inside his/her brain is called a Controller.
There are many planets Yeerks have conquered, and their next target is... the Earth!!

The kids never have a dream to become an Animorph, but coincidentally they met a dying alien, Andalite creature, named Elfangor. This Andalite gave them the power to morph into any animal they touch so that they can continue to struggle against Yeerk villains that are attacking the Earth.

Ever since that moment, their life as a kid have changed altogether. Not only have to face school matters and a pile of homework, they also have to deal with a mission to save the world from Yeerks' conguest.

The morph power is an Andalite's technology. So, the Yeerks think that the Animorphs are the Andalite army. No one knows that actually they are only a bunch of innocent kids. Animorphs are moving stealthily in secretive ways. No one knows that they have the power to morph, no one suspects that they are fighting against the cruel aliens, even their parents. In school, they pretend as if they don't know too close each other. They must be cautious if they want to meet, or just talk by phone. They can't ask for help to anyone, and believe anyone, because anyone might be a Controller...

Two hours is the maximum limit for them to morph. Exceed the limit, they will be trapped inside that animal body, forever....

Those are what make the Animorphs series become a wonderful and nice books to read. As the early adolescents, sometimes the Animorphs behave so rilex and full of jokes in facing their adventures, while truthfully they are very strained and scared. With their morph power, and ally with Ax — Elfangor's little brother — and a number of aliens who are opposite to Yeerks, they are fighting together wholeheartedly, for annihilating Yeerks.