When a disgraced Visser Four finds the Time Matrix and resolves to kill some of history's most influential people, Tobias and the other Animorphs must follow him through the ages to stop him before it's too late. Tobias is a boy who was trapped in hawk morph in a battle against the Yeerks. Now he works as a scout and sometimes-fighter, helping his fellow Animorphs to protect the Earth from a subtle invasion by mind-controlling aliens, the Yeerks. The Animorphs are badly outnumbered, but they haven't lost yet, and they're determined to keep fighting.
But Visser Four, a Yeerk who controls a man named John Berryman, finds the Time Matrix and decides to use it to remake human history. He plans to weaken humanity so that when the Yeerks finally do arrive, they can take over the world far more quickly, and won't have to deal with powerful human governments or good communications technology (such as the Internet) that might thwart their efforts.
Tobias and the other Animorphs get roped in because the Ellimist and Crayak don't want anyone changing history for selfish reasons. As Visser Four travels to Agincourt in the eleventh century, then to George Washington's military encampment during the Revolutionary War. Jake, the leader of the Animorphs, is mortally wounded during the river crossing, and when Visser Four travels on again, his body is left behind.
At Trafalgar, Tobias and the other Animorphs struggle to prevent Visser Four from successfully changing the course of the battle, but fail. Eventually, after witnessing several more events of great historical importance, they catch up with Visser Four a few miles from an alternate history version of the Battle of Normandy, where Tobias kills Adolf HItler, who is (in this version of history) a driver of military vehicles rather than a statesman.
Visser Four's host is severely wounded, and the Yeerk tries to flee but is killed by Marco. Cassie speaks to John Berryman, and learns the time and place of his parents' first meeting. By traveling to San Francisco in 1967, Tobias and the other Animorphs sufficiently distract Berryman's mother that she is never introduced to his father, and as a result, John Berryman himself is made to have never existed.
The end of the story has the Animorphs back at Cassie's barn, alive and whole. Jake, who had died, has been resurrected; since he had only died as a result of time travel, and the time travel adventures had been made to have never happened, his death was also undone. As a result, the Animorphs haven't gained ground, but they have saved the timeline, and they admit that this might well have been the best possible outcome.
Best part of story, including ending:
It was a fairly creative use of time travel technology. The narration was shared between the various Animorphs, which is interesting as all of them think quite differently from one another. I did like the way each era's worldview affected the Animorphs, with some people being shocked at their animal transformations, other people being shocked at their ages or physical appearances, or by the odd appearance of the Time Matrix once they secured it from Visser Four.
Best scene in story:
The first major historical event the Animorphs visit is the battle of Agincourt. When I was much younger, this was one of the first times I'd read about foot-soldiers dealing with horrible weather, and the imagery--horses slogging through mud, rain getting everywhere, people having difficulty hearing orders because of all the background noise of the storm--has stuck with me, years later.
Opinion about the main character:
Tobias was one of my favorite characters in the Animorphs series. He's fairly serious by nature, thinks tactically, and doesn't open up to many people, in part because of an abusive background. But he is fiercely protective of his friends, and generally stays level-headed even when everything around him is going badly.