It may behoove you to read the previous book in this series, Mossflower
In the early days of the Redwall series, it was common for Jacques to reuse some characters; though he tended to create more self-contained books later on, I think he wrote as he did in the early days because some of his original characters and lore were so endearing.
Mattimeo takes place a mere few years (though the animals measure time in “season,” four of which would, I assume, equal a year) after the events of Redwall. The main characters are still Matthias the Warrior, Constance the badger, Cornflower the mouse, &c. The difference here is that most of the characters have children now, and it is these children who form a third of the tripartite narrative—Mattimeo, son of Matthias, especially.
The basic plot is that a mutilated fox from Redwall, now named Slagar the Cruel, manages to kidnap some Redwall children, both in order to gain misguided revenge and to eventually profit from his slave trade. Predictably, the book is a bildungsroman for young Mattimeo, and to a lesser extent his enslaved companions. Meanwhile, while the young animals march south, a group of Redwall warriors goes on the hunt, picking up companions along the way. The third and final piece of the plot is the army of ravens that invades Redwall itself, whose advances must be checked by a ragtag group led by Constance the Badgermum.
Jacques weaves these stories with his usual aplomb. Expect your mouth to water at all the descriptions of food; expect a number of stoats, ferrets, and weasels to get stabbed, crushed, or otherwise killed in the course of battle. Expect the ghost of Martin to make an appearance, as he seems to do in most of the early books.
I admit to being a little underwhelmed by the villain in this one—not the mutilated fox, but the polecat who enters later. The entire climactic battle was a bit difficult to understand, which put kind of a damper on its excitement. So, too, the battle on the homefront, with the ravens, was solved with a bit of a deus ex machina and wasn’t as satisfying as I’d hoped. I recall being similarly disappointed with Mattimeo when I first read it as a young boy.