In early Britain, Sir Ontzlake of King Arthur’s court arrives at the Earl of Yeonil’s castle with news that Viking raids are continuing against Camelot. Before meeting Yeonil, Ontzlake greets respected sword maker John. Later in private, Yeonil and Ontzlake express unease over the motives of King Mark, who has offered an alliance with Arthur against the Vikings. Meanwhile, the earl’s daughter, Lady Linet, visits John in the forge and insists that their class difference cannot change their feelings for each other. Yeonil discovers John and Linet in an embrace and regretfully orders John from the castle with money and a horse. Ontzlake encourages John to win Linet by becoming a knight as Ontzlake did years earlier. Just after John follows Ontzlake to Camelot, the Vikings raid Yeonil’s castle, and John returns to defend the family, only to witness the brutal murder of Lady Yeonil and the earl’s savage beating by an assailant, who then laughs aloud in triumph. When Lady Yeonil’s killer and the Viking leader bolt from the attack, John gives chase, but Linet believes he is fleeing in fear. John sees the pair change into royal garb, and follows them to Camelot, where Arthur, Mark and the court are celebrating the Feast of Pentecost. John bursts in on the banquet with news of the attack on Yeonil and accuses Saracen Sir Palamides of leading the raid and his servant Bernard of Lady Yeonil’s murder. After Arthur reveals that Bernard is a deaf-mute and orders John’s arrest, Ontzlake reminds Arthur that at Pentecost, everyone is allowed a “boon,” or special request. Arthur grants John three months to prove his accusations against Palamides, threatening death should he fail. Later in private, Palamides and Mark discuss their plot to overthrow Arthur and take control of Britain. Palamides rebukes Bernard for carelessly speaking aloud during the attack, thereby jeopardizing his useful position at Camelot. Meanwhile, John accompanies Arthur to the ruins of Yeonil Castle, where they find Linet caring for the bewildered, beleaguered aarl. Linet is dismayed to see John and refuses to speak with him. The next day back in Camelot, Arthur sees Mark off and evades an assassination attempt arranged by Palamides. When John tries to join the knight training school, Palamides mocks him, spurring Ontzlake to tutor John privately. After weeks of training, Ontzlake presents John with a fine horse and the famed Sir Lancelot’s original armor, and cautions John to act with restraint against Palamides. Ontzlake then sends John off to make a name for himself in knightly confrontations. John fashions Lancelot’s old armor into a new black suit and, without divulging his identity, defeats one of Arthur’s newest knights, Sir Hal, in a jousting contest. Hal reports to Arthur of the Black Knight’s fighting prowess and regal bearing, prompting Palamides' suspicions. Later, when Palamides tries to provoke John to fight in front of Linet, John recalls Ontzlake’s caution and refuses, convincing Linet that he is a coward. Mark returns to Camelot for the celebration of the opening of the new church, but like Palamides, is apprehensive about the peoples’ enthusiastic support of Christianity. The ceremony is broken up by a Viking attack, secretly arranged by Mark, in which several monks are killed. John, as the Black Knight, rides off to summon Arthur’s knights, as Linet, the abbot and the remaining monks are taken by the Vikings to the mysterious rocks of Stonehenge, where they are readied as sacrifice. Arthur, John and the knights arrive in time to rescue Linet and the clergymen and destroy Stonehenge. Linet is overjoyed to learn that John is the Black Knight, and they are reunited. Later, in order to lure the Black Knight, Palamides kidnaps Linet and demands to know the Black Knight’s identity, suggesting that he led the Viking attacks. When Linet refuses to cooperate, Palamides orders Bernard to torture her. Learning of Linet’s kidnapping, the Black Knight comes to her rescue, arriving in time to hear Bernard shout when Linet attacks him with a knife, before fainting. As Linet cannot support John’s accusation that Bernard can speak, Ontzlake tells John that his identity as the Black Knight will soon be known and that he must get proof of Mark’s treason. With the aid of a sympathetic insider, John infiltrates Mark’s castle and overhears Mark and Palamides' plan for the invasion of Camelot while casting blame for the Viking raids on the Black Knight. Later, at Arthur’s court, Palamides presents Arthur with the Black Knight’s shield and accuses him of treason. When the Black Knight arrives in court, Arthur has him arrested and imprisoned, but Linet helps him escape. John tells Ontzlake about Mark’s attack plan, then slips into his castle and forces Mark back to Camelot. The dawn of the day of the invasion, Palamides orders Bernard to assassinate Mark after the attack commences. Bernard sneaks into Mark’s chambers and stabs the sleeping figure, then triumphantly declare Palamides' victory. Unknown to the servant, Arthur, John and the other knights are hidden in the room and hear his remark, which clears John at last. Arthur leads a counter assault on Mark’s attacking Vikings and banishes them. John is formally knighted by the king and receives Arthur’s pleased consent to wed Linet.