High King Uther banishes his son Arthur, who rescues a dying boy named Derfel. Arthur entrusts Derfel to Merlin and Nimue. Years later, Uther has a new heir, but Merlin has a dark vision for the future.
Arthur takes Gundleus prisoner. Nimue struggles with the trauma of her rape and plans to kill Gundleus. Arthur learns that Gundleus' uncle Gorfydd is preparing for war. Nimue tries to pull Derfel into her plans, but Derfel can't betray Arthur.
Derfel feels uncomfortable about the attack on the mine – which Owain is keeping a secret. Nimue is discovered by Merlin who helps heal her. Tristan arrives demanding retribution but Owain denies his guilt.
Arthur believes that Bedwin will be able to negotiate peace with Gorfydd; however, when Morgan and Derfel arrive in Powys, they discover a horrific site.
Derfel finds Nimue inside the Isle of the Dead; guilt ridden by the deal he's been forced to make with the Saxons, Arthur vows to fight to save Britain.
