Animorphs

Tobias, trapped in animal form after morphing into a hawk, agrees to help save two Hork-Bajir from the evil Yeerks in exchange for his humanity.

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.

Marco is a reluctant Animorph, having been given the ability, along with four other friends, to change into animal forms by an Andalite prince, an alien who wants help in stopping an invasion of the enemy Yeerks, but when he discovers that his mom, who he believed was dead, has been taken over by the Yeerks, he finds a reason to fight.

Cassie believes her strange dreams are just a side effect of having been given the alien power to morph into an animal, but when she learns that Tobias has been having the same dreams about the ocean, they realize someone is issuing a call for help.

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.

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.

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.

Marco and the other Animorphs discover that Visser One is overseeing a secret underwater project and Marco doesn't know if he can face his mother.