Mordred the warlock besieges Camelot, but Uther Pendragon defeats him with a magical sword forged by Merlin. Uther’s brother, Vortigern, covets the throne, sacrifices his wife to become a Demon Knight, and kills Uther. Uther’s son, Arthur, escapes to Londinium, where he grows up as a crime boss.

Vortigern rules cruelly and tries to retrieve a sword in a stone. Arthur, captured, pulls out the sword but faints. Vortigern reveals Arthur’s lineage and plans to execute him. A mage, allied with Merlin, rescues Arthur, who learns of his true heritage and Vortigern’s betrayal.

Arthur and the rebels fight against Vortigern, but face many challenges. Arthur nearly throws the sword away but is shown a vision of England’s future under Vortigern. He regroups with his allies, but they are soon captured.

Arthur negotiates with Vortigern, promising to surrender himself in exchange for his allies’ freedom. The mage aids Arthur with venom to protect him. A giant snake attacks the castle, but Vortigern survives due to snake blood protection. Arthur reclaims Excalibur, defeats Vortigern after a fierce battle, and restores order.

Arthur is crowned king, knights his friends, and begins building his Round Table.