Warning: Spoilers follow
The book revolves around the attack of Cluny the Scourge, an evil, infamous rat, and his horde, upon Redwall Abbey. Near the beginning of the book Cluny's horde arrives and makes the Church of St. Ninian its base, south of Redwall.
There are two main parts of the plot. The first is the various schemes that Cluny tries to use to invade the Abbey, and the efforts of the Redwall mice (and most of the creatures from the surrounding country who have taken refuge there) to repel these efforts.
The second part follows the quest to find the legendary sword of Martin the Warrior by Matthias, helped particularly by Methuselah. Matthias is a young Redwall novice. The start of the story depicts Matthias as a small but brave-hearted young mouse. As the story develops, the natural leadership qualities of Matthias begin to emerge.
The quest to find the sword is initiated by the discovery of some riddling writing on a wall in the Abbey. The clues eventually lead to the discovery of the location within the Abbey where the sword was hid. However, the sword is no longer in its hiding place. The futher search for the sword leads Matthias to the Abbey roof where the sparrows live, and then to the lair of the gigantic snake Asmodeus.
Meanwhile, Cluny has been attempting a number of unsucessful strategies to gain entrance to the Abbey. He finally persuades the dormouse Plumpen to treacherously allow him into the Abbey, and Cluny takes over Redwall.
However, Cluny's victory is short-lived. Matthias retrieves the sword from Asmodeus's lair. He, along with the allies whom he has met, returns to free Redwall and rid the land of Cluny the Scourge. There is a climactic dual betwen Matthias and Cluny.