Uther's daughter Morgana returns from banishment and poisons her father. Merlin arrives too late to save the king but gets Uther to acknowledge his illegitimate son on his deathbed. Merlin takes the young Arthur to the ruined citadel of Camelot.

Morgana faces opposition in her alliance with King Lot. Meanwhile, Arthur acquires the sword in the stone and is crowned King.

Morgan invites Arthur to a feast and Merlin accompanies him. While the temptress delays Merlin, Arthur returns to Camelot, drawn by his fascination with Guinevere. Meanwhile, Kay and Leontes seek out a famed warrior to recruit him to Arthur's cause.

Whilst Guinevere wants to leave Camelot with Leontes, he's obliged to stay as one of Arthur's knights being trained by brilliant strategist Gawain, who breaks Arthur's sword in combat. Having warned Arthur to resist his feelings for Guinevere, Merlin sets out to have a new sword made by reclusive bladesmith Caliburn, a widower living with his daughter.

Arthur and his knights arrive at a village where the head man has been killed and his alleged murderer Colfur is about to be lynched. Arthur insists his trial take place at Camelot as a precedent for a justice system to establish fairness. By getting Guinevere to talk to Colfur's young daughter, he finds that the case is not as simple as it first appeared.

Morgan impresses the supplicants who come to her court but is visited by a woman who claims Sibyl set fire to a convent in which her daughter died, requiring Morgan to punish the nun. Arthur accompanies Guinevere to visit her dying father, resisting further sexual temptations on the journey.

Despite the misgivings of Igraine and Kay, Arthur and his inner circle attend a banquet given by Morgan as a sign of reconciliation, during which her castle is attacked. Arthur's men prepare for defense but Merlin is suspicious as the 'attack' is evidently stage-managed by Morgan to break up the party.

Morgan fools Camelot with her disguise, even Merlin. However, after Redwald, a little orphan boy, whom she and Merlin have helped celebrate his birthday, accidentally dies because of her, Merlin has his suspicions. Vivian, feeling sorry for Igraine, releases her to confront Morgan but Morgan, in her guise as the queen, has let slip to Leontes his wife's indiscretions with Arthur.

Morgan makes her escape, leaving a puzzled Igraine believing she is going mad until Merlin explains Morgan's deception. They go to Castle Pendragon to confront her but end up as her prisoners. Leontes, now aware of his wife's infidelity, prepares to depart Camelot but joins Arthur and the knights in defending Bardon Pass.

Morgan enters Camelot with her prisoners, claiming that they're traitors and that only she can save the realm from the marauding bands. Arthur does a sterling job of holding the attackers at bay using booby traps until his men return to rout the invaders.