High King Uther banishes his son Arthur and welcomes a new heir. Cast out, Arthur finds a young boy, Derfel, close to death whom he saves. He leaves Derfel in the care of Merlin and his prodigy Druidess, Nimue.

Uthers health worsens. Gundleus attacks Avalon, with dreadful ramifications. Nimue and Derfel try to escape along with a group of other survivors. All appears lost, when a surprising figure reappears.

Arthur is forced to negotiate with hostile forces in the pursuit of peace. As Nimue struggles with the trauma of her attack, she strikes a pact with Derfel to be there for one another, whilst planning her revenge.

Derfel and Owain leave to collect taxes, but are diverted and end up raiding a mine. Nimue feels betrayed by Merlin. His leadership style in doubt, Arthur visits his childhood home with Morgan, and makes a surprising discovery.

Nimue takes her fate into her own hands. Tristan arrives at Caer Cadarn demanding retribution for the raid at the mine. Under Gorfydds watchful eye, Arthur has no other option than to intervene and faces a terrible decision.

Arthur suggests that as a show of faith, he marry Gorfydds daughter, Ceinwyn. He and Derfel travel to Powys along with Gundleus and Gorfydd. Arthur meets a fascinating and beguiling figure, before he and Ceinwyn are betrothed.