Elfangor's Secret (Animorphs Series: Megamorphs #3)

"We found out who Visser Four is. And he has found the Time Matrix, the machine Elfangor had hidden in the abandonedconstruction site. The same place we met him on a night none of us will ever forget. Especially me. Now Visser Four has theMatrix, and he plans to use it to become Visser One.But Jake, Rachel, Cassie, Marco, Ax and I can't let that happen. We can't let him alter time so that the Yeerks will win theinvasion. So we're prepared to make the ultimate sacrifice. Eventually, one of u...