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.

An untimely death leads Merlin to locate Arthur and urge him to come back to Britain; chaos ensues when Gundleus assaults Avalon.

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.

Nimue returns to Avalon, where she is found by Merlin. Derfel and Owain travel to collect taxes from King Cadwys. Gundleus returns to Caer Cadarn.

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 and the tribe kings enter peace talks. Arthur meets Guinevere, Ceinwyn's lady-in-waiting, while traveling.

Arthur and Guinevere meet secretly before Arthur and Ceinwyn are to be married.

Gorfydd allies with the Saxons and declares war; Arthur admits he exiled Nimue to the Isle of the Dead.

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.

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.