Mordred, an iron-fisted warlock portrayed by Rob Knighton, launches an onslaught against Camelot, aiming to enforce the supremacy of magic-wielding Mages over humanity. In a bold move, Uther Pendragon, the Briton king played by Eric Bana, infiltrates Mordred's hideout and decapitates him, leading to the disbandment of his armies and the salvation of Camelot. However, that very night, Uther's treacherous brother, Vortigern (Jude Law), who yearns for the throne, stages a coup. In a shocking act, he sacrifices his wife Elsa (Katie McGrath) to summon a demon knight, which results in the tragedy of slaying Uther as well as his wife. The sole survivor of this calamity is Uther's son, who floats away in a boat and finds himself raised by prostitutes in Londinium, where he earns the name Arthur (Charlie Hunnam).
Years pass, and Vortigern is now the king. As the waters surrounding his castle recede, they unveil Arthur's legendary sword, embedded in stone. Vortigern, desperate to assert his claim to power, summons the demon knight once more, who reveals the prophecy of the King’s return. The demon commands Vortigern to seek out young Arthur and retrieve the sword, which will only respond to its rightful master.
Growing up as a resilient street fighter, Arthur makes friends with Tristan (Kingsley Ben-Adir) and Backlack (Neil Maskell). The trio confronts a band of Vikings led by Kjartan, forcing them to compensate one of the prostitutes they had wronged. After a raid on the brothel by Vortigern's minions, Arthur discovers that the Vikings were actually guests of the king, and attacking them equates to a crime against the crown. Captured while attempting to flee, Arthur finds himself on a ship with many young men, coerced into attempting to draw the mysterious sword from the stone. When Arthur manages to pull it free, he is engulfed by its formidable power and faints. Vortigern confronts him in the dungeon, revealing the truth of Arthur's lineage.
A pivotal moment occurs when a woman known only as the Mage (Àstrid Bergès-Frisbey) approaches Uther's former general, Sir Bedivere (Djimon Hounsou). During Arthur's planned execution by Vortigern, the Mage employs her magic to create a distraction, enabling Bedivere's men, Percival (Craig McGinlay) and Rubio (Freddie Fox), to rescue him. At first, Arthur hesitates to join their cause, but when he attempts to wield the sword, he collapses yet again, leading the Mage to propose they venture into the “Darklands” — the realm where Merlin defeated Mordred's tower.
As the tale unfolds, it is revealed that Vortigern had incited Mordred to revolt against humanity, driven by envy of Uther’s power. With the sword's magic restored, Arthur resolves to confront Vortigern for his treachery. Teaming up with his friends and Bedivere's men, Arthur orchestrates several sneak attacks to flush Vortigern out from hiding. Aware that his maid Maggie (Annabelle Wallis) is spying for Bedivere, Vortigern concocts a plan to meet with his barons in Londinium, employing a body double to deceive the rebels. Though they realize the trickery, they proceed with the attack, which unfolds as part of Vortigern's greater scheme. In this chaos, Rubio is taken captive while Backlack sustains serious injuries.
Seeking refuge in a fighters' school led by Arthur's mentor, George (Tom Wu), they soon find themselves besieged by Vortigern's forces. When Arthur witnesses the Mage in peril, he channels his fury and unleashes Excalibur’s might, single-handedly vanquishing the enemy soldiers. As the night engulfs Londinium in riots, the rebels find sanctuary, joined by Backlack, who brings along his son, Blue ([Bleu Landau]). Mortally wounded, Backlack encourages the group to persevere, promising that he will follow them soon. However, unknown to them, Vortigern's commander has been tracking Backlack, leading to a tense confrontation where Vortigern and Mischief John extract critical information.
Arthur intervenes just in time to save Blue, but tragically, Blue witnesses his father's death. Overwhelmed by guilt, Arthur discards the sword into a lake and retreats into the woods. Encountering the Lady of the Lake ([Jacqui Ainsley]), he is drawn into her mystical realm, where he receives a haunting vision of England ravaged under Vortigern’s tyrannical rule. Recognizing his responsibility, Arthur reconnects with Bedivere, George, and Percival, returning to their hideout only to find it decimated, having fallen victim to Vortigern's betrayal and Rubio's torture.
Faced with an ultimatum from the king, Arthur must surrender himself by nightfall to save both the Mage and Blue. In a tense standoff, the Mage summons a gigantic serpent to wreak havoc on Vortigern's forces, as a desperate Vortigern transforms into the demon knight. In a climactic battle, Arthur initially falters but, through the memory of his father's sacrifice, he harnesses the sword’s power and strikes down Vortigern, blessing him as he perishes.
In the aftermath of this fierce conflict, Arthur, alongside George and Percival, begins the groundwork for the round table, symbolizing unity among his knights. He also dissolves Vortigern’s alliance with the Vikings, asserting that they must acknowledge England's strength to prevent future hostilities. Blue and the prostitutes present Arthur with Uther's crown, which he accepts gracefully. As the new King Arthur of England, he raises the sword triumphantly, ready to lead his people into a new era.
