In the kitchens of Redwall Abbey, Mattimeo, son of Matthias and Cornflower, fights Vitch, a mean little rat. Mattimeo is punished by his father for discourtesy to a guest. But Mattimeo's instincts about Vitch are validated when it transpires that Vitch is a spy for Slagar the fox - a slaver for the terrible underground Kingdom of Malkariss. Slagar gains entry to Redwall disguised as the leader of a troupe of magicians. At the great feast he is able to drug the drinks and capture Mattimeo and his friends: Tess, Tim, Sam Squirrel, and Cynthia Bankvole.

Redwall Abbey awakes to find their young ones are gone. As they piece the previous night together, they realized that Mattimeo was right that Vitch was the slaver's spy. Desperate that he ignored his son's warnings, Matthias vows to pursue Slagar the Fox accompanied by Basil and the others. He is unaware that Orlando is already in far away pursuit.

Mattimeo realizes that the slaves are headed south while their parents are following false trails. They must escape by themselves or not at all. It is a slow beginning of his transformation as a leader, but not yet in time to save them from the treachery of Scurl the frilled newt who steals their few belongings for false freedom.

Mattimeo and the slaves make their first attempt to escape. Mattimeo takes another step towards his birthright of leadership. Matthias and the others are now chasing them in the right direction.

Mattimeo and his companions race to the landslide beneath where their parents are buried. They are seen by Slagar who send rats to recapture them. Within the cave Matthias and the pursuers are close to suffocating while above them - unknown to them - Mattimeo digs frantically.

From Redwall Abbey, Warbeak and her sparra squadrons fly South bearing the riddle. They hope to find Matthias to help him for his quest. But they are unaware that their departure is observed by Ironbeak, an evil raven general who invades Redwall as soon as Warbeak is gone. They drive the citizens of Redwall into the cavern hole and make them prisoners in their own abbey. Slagar and his slaves ascend the huge Great South Cliffs and cut off the means of pursuit leaving Matthias unable to follow. They pass along through the Dark Forest and are attacked by the fearsome and terrifying Painted Ones. They survive to find that Slagar is about to cross a huge river beyond which a huge rat army awaits him and his slaves.

Safely across the river, Slagar awaits Stonefleck's army of rats. Below the cliffs, Matthias meets Sir Harry the owl who reveals a way to ascend them. Matthias and company ascend but also comes under attack in the Dark Forest almost losing Cheek before making their escape. Across the river Mattimeo leads an escape of his own but is immediately recaptured by Stonefleck's rats. Back at Redwall, Ambrose Spike and the others set traps for the invading birds and they engage in what is to be a long battle of wits and courage. The birds take Cornflower, Rollo, and Mrs. Churchmouse hostage and threaten to kill them unless Redwall surrenders. The stand-off temporarily resolved by the Abbot, but it leaves the captives in great danger. Matthias and the pursuers launch a raft to rescue Mattimeo but come under massive fire and take to the water, only to find it is alive with nibbling fish. Without any choice, they have to ride the river.

Matthias's raft drifts towards the rats on shore, watched in horror by Mattimeo and friends. In the roof of Redwall Abbey Baby Rollo, Mrs. Churchmouse, and Cornflower wait to be thrown down unless the abbey surrenders. Triumphant, Slagar drags the miserable slaves onwards as Warbeak desperately and unsuccessfully still seeks Matthias.

Slagar reaches the abyss and forces the slaves across a rope bridge, cutting it away behind him. There is now no way for Matthias to pursue him. A full day and night behind the pursuers arrive at the abyss and are stopped dead, until Sir Harry comes their rescue by retrieving enough rope to cross the bottomless gorge.

As the slaves arrive at the ominous Badger Head and Bell rocks, Mattimeo bravely comfronts them and promises "I will not fail us." At Redwall the citizens defend the tapestry in a pitched battle and recapture it. Simultaneously Matthias arrives at the Loamhedge Abbey and meets an old rabbit who forecasts doom, giving him an amulet.

Underground in the kingdom of Malkariss, Mattimeo and 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 dark cell. Matthias unravels the last secret of the Loamhedge parchment and and finds a way into Malkariss - the old rabbit's amulet is the key to enter!

In the kingdom of Malkariss, Matthias fights the slavemaster, Wearet, and falls into the abyss. There he grimly hangs on the rope that previously lowered a mysterious basket. But as Mattimeo and friends are released by the dying Log-a-Log, the Wearet cuts the rope and Matthias falls into the basket - and meets Malkariss himself!

In the pit of the kingdom of Malkariss, Matthias fights off Slagar who escapes to daylight as Matthias joins his battling friends - now reinforced by the released slaves. As Matthias joyfully embraces Mattimeo an earthquake shakes the ground. The terrible Polecat statue topples and with it the kingdom of Malkariss starts to collapse.