In a crypt below Camelot, Nimueh performs necromancy on a sealed sarcophagus, her incantation ends with "Uther Pendragon." The stone sarcophagus cracks and a fist clad in armor punches through from inside as a delighted Nimueh beams at her creation.
At Camelot, King Uther leads Arthur's coming of age ceremony. Arthur swears the oath to protect Camelot and its people through life and limb as long as he shall live. Uther crowns him "Prince of Camelot." As if timed to test his oath, a knight in black armor crashes through the window on a black horse. Arthur draws his sword as the knight, his face concealed by his helmet, silently approaches. The knight throws down his gauntlet, which Arthur moves to pick up. However, Sir Owain moves faster and accepts his challenge. The knight states the rules, "Single combat. Noon tomorrow. To the death."
Merlin asks Gaius if he knows who the knight is. Gaius lies that he doesn't, claiming not to have seen the crest on his shield. Meanwhile, Morgana tries to convince Arthur to stop the duel or fight in Owain's place, but bound by the knight's code, he can't do either. Arthur explains that Owain picked up the gauntlet, so he's the one who has to fight. Something that Owain was well aware of when he made the move, Arthur tells Morgana. Both are very concerned that the fight is to the death.
Gaius goes to Geoffrey of Monmouth to confirm that the crest on the Black Knight's shield is indeed that of Tristan Dubois, the only knight known to have carried that crest. When Gaius tells Uther, he says that he also recognized the shield. However, Uther doesn't believe it's Tristan Dubois, as he killed him twenty years ago. Clearly spooked, Uther states more for himself than Gaius, "Dead men do not return."
The next day, Arthur coaches Sir Owain on dueling an opponent he's never seen fight. He advises Owain to quickly gauge his technique, to which Owain says he has the same advantage. The Black Knight hasn't seen him fight either. Owain asks Arthur his opinion on his skills, to which he replies he's seen no one braver. Gwen (Guinevere) enters with a red ribbon from Morgana for Owain to wear for luck. Thanking her, he says he'll wear it with pride but is confident he won't need luck.
Before a packed arena, Sir Owain enters with Arthur, who offers one last tip to his protégé in facing his mysterious opponent. Owain and the Black Knight engage in a fierce battle, during which Arthur yells out, reminding him that one well-aimed blow can fell his opponent. Owain lands it, stabbing the Black Knight deep in the abdomen. Everyone, including Owain, is stunned when he doesn't even wince at a wound that should have been fatal. They go another round, the Knight keeling Owain on his back with a brutal hit and wastes no time impaling Owain through the heart.
Before a fuming Arthur and stunned crowd, the Black Knight issues another challenge. Arthur leaps up, but Uther holds him back. Another knight, Sir Pellinore, accepts. Afterward, Merlin asks Gaius if they should tend to the Black Knight's wounds. Gaius seems confused until Merlin says he should be dead from the hit he took. Gaius suggests that perhaps he already is. Later, Arthur confronts Uther about holding him back. Uther says the other knights should get a chance to prove themselves. Arthur says Pellinore isn't completely healed from a previous battle and accuses Uther of sending him to his death. Uther denies that, saying Pellinore accepted the challenge.
Merlin and Gaius visit Tristan's sarcophagus and see someone's broken out of it. Back home, Gaius tells Merlin that Tristan is the brother of Igrayne, Uther's wife and Arthur's mother. Tristan blamed Uther for Igrayne's death during childbirth and traveled to Camelot to challenge him to single combat. Uther killed Tristan, who in his dying breath cursed Camelot to one day suffer his return. Gaius suspects the Black Knight is a wraith, a demented soul brought to life by powerful magic that harnessed its grief and rage. Gaius says a wraith can't be harmed by mortal weapons since it's already dead, nor stopped until achieving what it came for -- revenge.
The next day, Sir Pellinore fights the Black Knight, also managing a lethal stab in the gut. Morgana and Guinevere see the blow and think Pellinore has won, but the Knight shows no sign of injury. He fatigues Sir Pellinore, finally skewering him through the middle. Before the Knight makes another challenge, to Uther's shock Arthur throws down his gauntlet and issues his own. The Knight accepts. Afterward, Uther wants Arthur to revoke the challenge. Arthur refuses as it's against knight's code and can't prove he's worthy of the throne by being a coward. Uther says he'll die, which Arthur interprets as Uther having little faith in him.
While Merlin tries to find a way to kill the wraith with magic, Gaius visits Uther and reveals that Tristan's tomb is empty. Gaius believes Tristan's been conjured from the dead and seeks revenge for Igrayne's death. Uther says magic killed her, not him. Nevertheless, he blames Uther. Gaius advises Uther against letting Arthur fight, since mortal weapons can't kill a wraith. Uther says Arthur won't listen to him, prompting Gaius to suggest he tell Arthur who the knight is. Uther refuses to tell Arthur the secret of his birth, which only he and Gaius know. He warns Gaius not to break the oath of secrecy he took, reiterating that Arthur must never know.
Cloaked by the darkness, Merlin sneaks out to where the Black Knight stands in the courtyard at "rest." Merlin casts a spell that conjures a ring of fire around the knight. However, the fire has no effect on the wraith. At that, Merlin goes to Arthur and pleads with him not to fight because the knight will kill him. Arthur, annoyed that everyone thinks that, doesn't want to be viewed as a coward. Merlin says he's not a normal knight, pointing out that he doesn't eat or sleep, just stands there, but only succeeds in furthering Arthur's resolve.
Nimueh visits Uther to gloat at his predicament. She says he brought it on himself by banishing her from court and slaughtering so many of her kind. Uther said she practiced evil. She reminds him that he invited her to Camelot to give his barren wife a son. Uther accuses her of killing Igrayne. Nimueh says that wasn't her choice but the law of magic, of which Uther knew. To create a life, a death must occur to preserve the balance of the world. Had she foreseen the depths of his revenge, she wouldn't have granted his wish. Before vanishing, she says she's watched so many loved ones die at his hands, now it's his turn.
Merlin goes to Geoffrey's library under the pretense of looking for a book for Gaius. He explains that Gaius thinks the knight is a wraith and asks for his help in finding reference to weapons that can slay the undead. Geoffrey finds a story about a sword begotten in the dragon's breath.
Fueled by having a plan, Merlin visits Gwen to ask for the strongest sword her father's ever made to save Arthur. She gives him her father's prized creation; even though she knows he'll be angry at her for it. Merlin says he'll understand, as she did it for Arthur. Meanwhile, Morgana unsuccessfully tries her hand at convincing Arthur not to fight. Seeing he's unwavering, she tells him she understands.
Uther visits Gaius to apologize for his behavior, also admitting that Gaius was right so many years ago when he said no good would come from using witchcraft to gain an heir. Gaius offers comfort in that Uther couldn't have foreseen this, or Igrayne's death. Uther says he's going to fight in Arthur's place tomorrow since the wraith seeks revenge against him and not Arthur. Knowing he's going to die, he asks Gaius to keep his oath of secrecy about Arthur's birth. Gaius swears he will. Uther then asks his old friend for one more favor.
Merlin asks the Great Dragon to burnish the sword to save Arthur. However, when he explains the wraith has come for Uther, the Dragon says to let it take his vengeance and it will die without his aid. Merlin reveals that Arthur's fighting him, and if he dies Camelot will have no heir and Merlin no destiny. The Dragon warns that a weapon forged with his assistance will have great power, and makes Merlin promise it will be wielded by Arthur, and Arthur alone. In the wrong hands, it could do great evil. Merlin swears, and the Dragon forges Excalibur, afterward reiterating it's for Arthur alone.
At Uther's request, Gaius drugs Arthur by insisting he drink a potion to help him sleep. It's really a knockout potion to make Arthur sleep through the challenge, allowing Uther to take his place without incident. After Arthur succumbs, Gaius locks him in his room.
The next morning, thinking that Arthur is fighting the wraith, Merlin takes the sword to the armory. Upon admiring it, Uther enters and sees it, saying it's a fine blade. When Merlin says it's for Arthur, Uther says he'll use it, as it won't make any difference. However, upon holding it in his hand, he says it's worthy of a king. Despite Merlin's clever attempts to get it out of his hands, the king won't surrender it. When he learns that Merlin had it made for Arthur, he's touched by Merlin's loyalty to his son. He tells him to take care of him.
When Uther marches into the stadium instead of Arthur, the crowd is stunned. Uther challenges the wraith, saying it can have what it came for: the father, not the son. They battle ferociously, Uther unaware he has a magic sword. Meanwhile, Arthur wakes and upon hearing the duel, desperately tries to get out of his room. During the fight, Uther knocks off Tristan's helmet, revealing the wraith, a hideous decayed corpse. Recovering quickly from the shock, Uther makes his move when the wraith's sword gets stuck in Uther's shield, and runs him through. The wraith screams in rage and explodes into pieces. Furious, Nimueh stalks away.
Arthur's fury at Uther for drugging him is replaced by surprise that he did it because he holds him in such high regard. Arthur always felt like a big disappointment. Later, Merlin denies enchanting the sword when Gaius asks, having heard from Uther that Merlin provided it. The Great Dragon summons Merlin to ask if the sword worked, only to become enraged that Merlin broke his promise and let Uther wield it. He says it will do great evil in Uther's hands and tells Merlin he can't trust him, nor can he destroy what's been made. He says to hide it far away where no mortal will find it. Merlin takes it to the lake and casts it into the water, where it sinks to the bottom.