Arthur is King and Gwen is his Queen. But Morgana, his half-sister, is still a threat and does everything she can to thwart his rule. Morgana captures Gwen and holds her in a tower, subjecting her to psychological torture so that Gwen looks to her as a friend--and will do anything for her, including kill Arthur. And that's what she tries to do the next episodes--and Merlin knows it's her. I found these episodes a bit tiresome. Uther's ghost comes back and he's unhappy of how his son Arthur is running things, and there's an unpleasant face off between them. And Mordred, who is seen in a vision by Merlin as the one who kills Arthur, becomes one of the Knights of the Roundtable for a short time, until something happens that turns him against his new King. For the most part the story follows the King Arthur Legend.
I was surprised by a few things, particularly what happens to Arthur. And the final ending is cool, as we find out what eventually happens to Merlin, which is fitting. Conceivably, they could make a followup series which takes place in modern day.