Jake, Rachel, Cassie, Tobias, and Marco share an adventure in which their unusual abilities to transform into animals help them to solve a strange otherworldly mystery.

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.

David, the newest Animorph, is not what he seems to be.

The fact that his mother has been infested by an alien parasite and his father is planning to remarry puts Marco under such stress that he begins to morph into bizarre creaturees that he is afraid will attract the attention of the Yeerks.

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.

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.

Marco is determined to save his father from the Yeerks.

Ax and the Animorphs face the problem of deciding whether the time is ripe to go public with the facts of the Yeerks invasion and their resistance movement.

The arrival of Andalites on Earth gives Ax and the Animorphs allies to fight against the Yeerk invasion but also causes new poblems.

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.