Its about a controller named Henry, who used to be visser 4 but got demoted. He finds the Time Matrix and uses it to go back to important scenes and battles in earths history e.g. American War of Independece and D-Day. He then changes tries to change the outcomes of these battles so that the humans become a weaker species and easier to conquer. The ellimist and the Drode strike a truce in their war against each other to make the animorphs go to whatever time and place former visser 4 goes to stop him messing up the timeline. But the Drode wants something in return.
I read these books when I was in late elementary school through 7th grade. I am posting all of them which I own. When I was younger I really enjoyed them.
We found out who Visser Four is. And he has found the Time Matrix. The machine Elfangor had hidden in the abandoned construction site. The same place we met him on a night none of us will ever forget. Especially me. Now Visser Four has the Matrix, 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 this invasion. So we're prepared to make the ultimate sacrifice. And, ultimately, one of us will lose this fight...