Animorphs

Ax, an Andalite warrior-cadet, is stranded on Earth when his spaceship is attacked by the evil Yeerks, and, although he has misgivings, he agrees to join forces with five humans who have been given the power to morph into animals to help him fight the Yeerks and avenge his brother's death.

Rachel and her fellow Animorphs believe they have a real chance at defeating the Yeerks when they discover the location of the alien regeneration pool, but their plans are disrupted by an all-powerful Ellimist who offers them an opportunity to go live on another planet.

The Animorphs are in real trouble when they transform into flies to invade the new base of the alien Yeerks, and Jake becomes the enemy after falling into the Yeerk regeneration pool.

Tobias has stayed in his morph too long and must now live out the rest of his life as a hawk, but that does not keep him from contacting his friends and continuing the fight against the Yeerks, a race of hostile aliens that are invading and controlling the brains of humans.

Rachel, one of the Earth teenagers dedicated to stopping the parasitic alien race known as the Yeerks, is horrified when she finds herself split into two nearly identical girls.

The Yeerk that controls Jake's brother Tom is desperate to keep Tom from going with the family to his great grandfather's funeral, even if he has to kill Jake's father to do it.

When Marco's friend David finds the morphing box, he puts all of the Animorphs in danger by offering to sell it to Visser Three, and now the group must decide if they should let David die, or allow him to become the new Animorph.

When the Animorphs try to stop the tiny alien Helmacrons from taking the morphing cube, Marco gets in the way and ends up with Helmacrons up his nose and inside his head.

The Yeerks are working on a drug that saps humans of their free will but the Animorphs show them that human free will runs deeper than any drug can reach.

Jake and his friends are given the power to morph into animals by a dying alien who tells them the Earth is under attack by the invading Yeerks, and they are called upon to test their new abilities almost immediately when they learn that Vissar Three, deadliest of the Yeerks, is after them.