High King Uther banishes his son, Arthur, who rescues a dying boy named Derfel; Arthur entrusts Derfel to Merlin and Nimue; Uther has a new heir, but Merlin receives 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, and Nimue begins her revenge, Arthur learns that Gundleus' uncle Garfydd is preparing for war.

Bastard son Arthur returns to the land he's been banished from to find a kingdom in chaos. Seeking the aid of his oldest confidants, Arthur aims to right Britain's path as he faces the most unexpected of obstacles.

Derfel feels uncomfortable about the attack on the mine - which Owain is keeping a secret. Nimue aborts her pregnancy, and then 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 suggests that as a show of faith, he marry Gorfydd's daughter, Ceinwyn. She is horrified by the news. Arthur and Derfel travel to Powys and rest for the night by a lake.

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

Arthur believes Bedwin will be able to negotiate peace with Gorfydd. However, when Morgan and Derfel arrive in Powys, they discover Bedwin being crucified. A Christian mob accuses Nimue of poisoning them. They demand justice from Arthur.

Derfel and Morgan take Bedwin's body back to Arthur. Ceinwyn reveals that Gorfydd is allying with the Saxons. Derfel discovers that the Christians died of natural causes, but Arthur reveals he has 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.