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.