The Illusion (Animorphs #33)

There is an Anti-morphing ray out there. It's purpose: to cause a person in morph to revert to his natural form. And the Yeerks have done a very good job in hiding it. Word from the Chee that the Yeerks are ready to test the weapon. Just one problem: They don't have a test subject. Yes, they could have Visser Three go into morph and them hit him with the ray. But Yeerk scientists say
there's a definite chance that the ray will prove fatal. The Visser isn't volunteering.

The Animorphs come up with a clever and exceedingly dangerous idea: To find and destroy the weapon, they will supply the test subject. They will deliberately allow the Yeerks to capture one of them, follow the captured person, and learn where the weapon is. The dangers are obvious. If they are too late, the Yeerks will use the device to forcibly demorph the subject and reveal his true form: Human. Ax points out that he should be the one to volunteer. The Yeerks will discover an Andalite. Just what they expect.

Then Tobias has an even better suggestion. He should go. The Yeerks can take him, assuming him to be a hawk. They'll try their ray and nothing will happen. He'll already be in his true form. The Yeerks will think the weapon doesn't work. There's just one other problem. The Yeerks will hold Tobias beyone the two-hour limit. So in order to play the part of an Andalite in morph, Tobias will have to demorph to Andalite at some point. Only one solution: Tobias must acquire Ax...