The Illusion

The Yeerks possess a weapon that could be the biggest threat to the Animorphs yet. The anti-morphing ray transforms a person in morph back to natural form. Unless they can find and destroy the top-secret ray, the Animorphs could be exposed for good.
Tobias and the other Animorphs discover that the Yeerks, a parasitic alien race bent on enslaving Earth, plan to test a device that could put an end to the Animorphs' fight against the Yeerks.