Arthur Pendragon, High King of the Britons, dreams of reforming the diocesan council at Camulod. The council has not met since the days of Vortigernos. Arthur intends to even extend it to include the British kingdoms beyond Hadrian’s Wall. However, with the Saxons defeated at Badon, many of the British kings have already resumed their old rivalries. Some would rather deny any authority above their own. Others resent the ideals Camulod stands for as counter to their own. Still others would rather see the young High King destroyed before he has a chance to establish his power.
While he sees to the growing tensions with Icel, Arthur sends his knights to prove Camulod’s worth to the various kingdoms of Britain. But before they leave, Myrddin calls them to the Giants’ Dance to tell them about the Treasures of Britain — twelve treasures from British legend and history that, collected together, could replace the protection once offered by the head of Bran the Blessed, and keep Britain safe from invasion. They already have one: Caliburn. They need to find and collect 11 more, including the Horseman’s Knife, the location of which is known only to Vorcunos. Lugh challenges Myrddin to find another treasure, to respresent Arthur’s contribution to British history. He is intrigued by the idea, and agrees.
The knights visit Vorcunos in prison. Vorcunos tells them the knife is hidden in a cave in Eboracum, but cannot — or will not — tell exactly where and insists on acting as their guide. Lugh can tell he’s lying and plans to escape, but that he also still has some humanity struggling to get out. Peredur orders better lodging and treatment for his father, to coax his humanity back. But they don’t want to go to Eboracum just yet. They decide to begin in Dumnonia, where Arthur is already well-liked by the leadership and the Mantle of Joseph of Arimathea is hidden.
Marcus Cunomorus welcomes them warmly to Isca Dumnoniorum, the capital of Dumnonia. Peredur drinks with the locals and finds that the common folk hate the nobles and have great love for a man named Gerontius, who is raising a rebellion against Cunomorus, and a pirate named Bellinios. Meanwhile, Caius and Bedwyr find out that a hermit in the countryside named Petiacos knows where the Mantle is hidden.
At the feast that night, Cunomorus tells them of a sea monster called the Morholt that lives between Dumnonia and Ireland and has been attacking trade ships. He asks the knights to kill it. Lugh forces him to agree that if they kill the Morholt, Cunomorus will treat his people better — if only for the sake of stability — and enforce Arthur’s new laws in his land, revoking many of the old Roman laws, like those binding farmers to wealthy landowners as serfs.
The knights obtain a boat, crewmen, and some harpoons and sail out into the sea, where they find and kill the Morholt — a giant, man-eating squid. Lugh, having been yanked into the water, lands the killing blow with his sword.
They meet with Gerontius, but all he wants is for Cunomorus to be deposed, and will accept no compromise. So they go to visit Bellatus, a Dumnonian sub-king, at Din Tagell. He asks them to remove the pirate Bellinios from his island fortress at St. Michael’s Island in Armorica, so that Bellatus’s men can seize it. They go there, and Lugh challenges Bellinios to a duel while Peredur and Bedwyr sneak around back, ready to attack in case things turn sour. However, Bellinios honorably agrees to the duel. Lugh does severe damage to his leg, but convinces him to peacefully abandon the fortress. The knights thus earn the loyalty of Dumnonia, but don’t feel very good about it.
When they return, they visit Petiacos and ask him about the Mantle. To prove that they are truly the friends of the Dumnonii, he tasks them with serving Bellinios for one month.