In a ruined castle, a beautiful seeress uses magic to observe Arthur through an enchanted crystal. She comments Arthur has a great destiny, and that the battle for his soul begins.
At Camelot, Uther and Arthur are bestowing knighthoods to their men, when a mysterious warrior enters the city, and fights their way to the castle, cutting down five guards with ease. The warrior fights their way into the Great Hall and throws down their gauntlet, challenging Arthur. Arthur accepts, and requests the warrior identifies themself. The warrior removes her helm, revealing herself to be the seeress, and says her name is Morgause.
In the aftermath of the incident, Geoffrey of Monmouth is unable to find any rule that states a woman cannot issue a challenge, and by the knight's code, Arthur is obliged to fight. Arthur is confident, but Uther warns him not to underestimate her. Morgause's presence also seems to unnerve Morgana, who claims to have a vague recollection of her. Later that evening, Merlin is sent by Arthur to ask Morgause to withdraw, since he has no wish to kill a woman in combat, but if he withdraws, it will be viewed as cowardice: however, Morgause refuses.
The duel is fought the next day: Arthur initially has the upper hand, even managing to wound Morgause on the arm, but she eventually overwhelms him. Left lying on the ground with Morgause's sword aimed at his heart, Arthur is offered a reprieve: she will spare his life if he agrees to meet her outside Camelot in three days time and accept whatever challenge she lays down. Arthur reluctantly agrees. Afterwards, Morgause's wound is treated by Gaius, who also appears to have some memory of her. When he asks, Morgause says she has only been to Camelot once before as a baby and that he probably won't remember her: however, Gaius seems to recognise a bracelet on her arm.
Morgana goes to Morgause's chambers to introduce herself: however, Morgause claims to know who she is. She denies they have ever met, but asks to be remembered fondly and offers Morgana the bracelet she wears as a gift, saying it has healing powers and will help with her nightmares: however, Morgana refuses. However, that night, Morgause leaves the bracelet beside Morgana's bed while she sleeps and casts a spell. Morgana wakes the next morning, having slept better than she has in a long time.
Morgause departs Camelot the next day, reminding Arthur of his promise, and complimenting him as a man of honour...a trait inherited from his mother. However, Uther refuses to let Arthur leave, fearing Morgause plans to lure him into a trap. Though Uther confines Arthur to his quarters, he escapes with Merlin's help and sets off with him. The next morning, Gaius discovers Merlin and Arthur are gone and informs Uther, who immediately dispatches search parties to find Arthur.
Later that day, Gaius goes to give Morgana her sleeping draught, only to find her condition better, thanks to Morgause's bracelet. He later goes to Uther, telling him that the bracelet is marked with the emblem of the House of Gorlois, and that save for Morgana, only one other would wear such a thing: her half-sister. Gaius confesses he had smuggled her out of Camelot as a baby and given her to the High Priestess of the Old Religion, who will have likely trained Morgause in great and powerful magic: he also explains he never spoke of it because of a solemn oath, which he is breaking only because he fears Morgause's intentions. Uther insists Morgana must never learn she has a half-sister: he won't have her loyalties divided.
On their journey, guided by Arthur's horse (who was enchanted by Morgause to lead the way), Arthur explains he knows nothing of his mother: she died moments after his birth and his father never speaks of her. Merlin confesses a similar situation regarding his own father. They eventually cross a great lake and come to the ruins of the castle Morgause inhabits, where she is waiting for him. She challenges Arthur to put his head on an executioner's block and let her take his head, but she relents at the last moment: it was merely a test to see if Arthur would keep his word. As reward, she offers Arthur a chance to speak with his mother. Merlin is uneasy about Morgause's magic, but Arthur comments she has done them no harm so far, and remarks perhaps his father's belief all who use magic are evil is wrong.
Using her magic, Morgause stops time and allows Arthur to speak with his mother, Ygraine. As they talk, she reveals that Arthur was born of magic: that Uther, desperate for an heir, went to Nimueh and begged her to give him a son through magic, that he gained an heir at the cost of his wife's life. Furious, Arthur returns to Camelot and confronts his father, remarking that his hate for magic is caused simply because of his selfish greed and wish to blame someone else for Ygraine's death instead of himself. Arthur angrily derides Uther as a hypocrite and liar and attacks him: a ferocious sword fight erupts between the pair. Just as Arthur overpowers his father and prepares to kill him, Merlin and Gaius burst into the chamber. Desperate to stop Arthur, Merlin claims what they saw was simply an illusion conjured by Morgause to turn Arthur against his father and destroy Camelot (a claim Uther agrees with). After Uther swears it is a lie, Arthur relents, and both father and son break down in tears.
In the aftermath of the incident, Uther personally thanks Merlin for his actions, saying he has been a trusted ally in the fight against magic, but warns him he will have to be vigilant against sorcerous foes who will want to exploit Arthur's inexperience: he also has Merlin promise to tell no one of what happened on pain of death. Afterwards, Gaius remarks that Merlin must have been tempted simply to allow Uther to die: Merlin agrees, but replies Arthur would never have forgiven himself if he'd killed his father. Back at her castle, Morgause again watches Arthur, but her triumphant expression turns to fury when she sees they are still on good terms.