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.
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.