While Redwall prepares for a long siege, Cluny steals the symbolic tapestry of Martin The Warrior from within the Abbey, and Matthias learns that Martin's great sword still exists - but nobody knows where. As Cluny's next attack is repulsed Matthias meets a great ally, Basil Stag Hare, and learns that his own destiny is inextricably tied to his hero, Martin. Matthias will become Redwall's next Protector - but only if his quest for the sword is successful.
Cluny uses the treacherous fox, Chickenhound, to plant a rumour of an attack on Redwall, while Matthias and his mousemaid friend Cornflower unravel a mysterious motto - "I am that is". It is an anagram of "I Matthias" written years before he was born - another clue on the quest for Martin's sword. Cluny's real underground attack is inventively repulsed by hot porridge and Matthias sees the fearful snake Asmodeus for the first time and realizes that somehow their fates are horribly intertwined.
Following complex clues, which promise to lead to Martin's Sword, Matthias climbs high into the Abbey roof - and the kingdom of the crazy King Bull Sparra. There he discorvers that the slithering Asmodeus has Martin's Sword! The King returns in time to fight Matthias who narrowly escapes death. But at last Matthias knows where his sword quest must end; in a battle against the Snake!
A travelling circus comes to Redwall and Cluny uses it to try and infiltrate the Abbey with his lieutenants, dressed as clowns. The mysterious circus star, Wild Ivy the Knifethrower seemingly mesmerizes Matthias, but she is, in fact, an ally and as Cornflower is threatened with death by Cluny, Ivy turns the circus against the rats in an acrobatic battle of fire-eaters versus fur! It is then we see what Wild Ivy has seen - that Matthias and Cornflower are destined to be together...if Redwall survives.
Cluny attempts to starve the Redwallers into submission. When Basil and Jess the Squirrel seem to desert, Matthias wonders if surrender is close. He decides to try and rescue the stolen tapestry from Cluny in order to give courage to the Redwallers - and discovers Basil and Jess attempting the same thing! They outwit Cluny, retrieve the precious tapestry of Martin, and renew the spirit of Redwall.
While Cluny plans to recapture the tapestry by burning down the Abbey gates, Matthias determines to renew his search for Martin's sword. He must find Asmodeus' hideout. He learns that only the fierce mouse-eating owl, Captain Snow, knows where it is. To ask him Matthias has to pass Julian Gingivere, a marmalade cat and the Shrew of Mossflower Wood! Matthias breaks off the search for the sword to return with his new allies to once again beat off Cluny.
Underground in the kingdom of Malkariss, Mattimeo and his friends see the slaves building a huge city. Brave Tess refuses to be a slave and Mattimeo is brutally attacked when he tries to save her from punishment. The slaves are thrown into a dank cell. Matthias unravels the last secret of the Loamhedge parchment and finds a way into Malkariss - The Old Rabbit's amulet is the key to enter! Redwall Abbey is also successfully fighting back utilizing the ruse of Martin's ghost and adds a new friend to its ranks in the form of Stryk, an enormous but injured kite-bird who ...
Cluny captures a travelling spice salesmouse and holds his wife. The fearful husband is forced to enter Redwall and take Cornflower captive in return for his wife. Torn between love and honor, he does as Cluny demands. But with Matthias's help, the two outwit Cluny and rescue the mousemaids, riding back to safety in a spectacular underground waterborne escape.
Matthais's wild sparrow friend, Warbeak, attempts to scout Cluny's camp as he prepares yet another attack on the Abbey. She is captured and Matthias has to rescue her before Cluny feeds her to Asmodeus the Snake. Posing as Cluny, Basil Stag Hare helps rescue Warbeak and Matthias resolves once more to find a way to kill Asmodeus. He stands between Matthias and Martin's Sword - their battle is inevitable.
Matthias sets off to find Asmodeus and recapture Martin's sword. Helped by the shrews of Mossflower he finally meets his foe in the deep caverns where he guards the Warrior's sword - the object of Matthias's quest. In a terrible battle Matthias kills Asmodeus. At last the serpent is dead and the sword is his. Now he can truly be the protector of Redwall. But news of Cluny's assault reaches him as he realizes he is trapped deep underground.
As Matthias is lost deep in Asmodeus's caverns, Cluny digs his way under Redwall, taking advantage of the exhausted Redwallers and the absence of Matthias. Using the sword to escape the caverns and after a terrifying river adventure, Matthias returns with the shrews to defend Redwall - but not before he is almost trapped by Cluny and only just saved by Cornflower. Cluny flees at the sight of Martin The Warrior's "ghost" while Matthias at last gets to wear his hero's armor.
Cluny seizes the family of a dormouse and threatens to kill them unless their father, Plumpen, betrays Redwall and opens the gates to his huge army of rats. The plan works and Redwall is finally captured. But Matthias challenges Cluny and in a final confrontation kills him. Cornflower and Matthias are at last married and Redwall returns to its idyllic lifestyle. Matthias has become the hero he admired in his childhood.