For centuries, humans and mages coexisted in peace. However the leader of the mages Mordred decides he wants more and sends his army to attack the kingdom of Camelot. Hundreds of mages together with giant war beasts launch a very aggressive attack, and no matter how hard they fight, Camelot's defenses aren't strong enough.
Prince Vortigern, who seems to feel dizzy for being close to magic, tells his brother King Uther that they should surrender, but Uther refuses. With his legendary sword Excalibur in hand, he meets face-to-face with Mordred, and the power of the sword is enough for Uther to defeat his enemy. Later, Uther and his council have a meeting about what they should do with the mages.
Sir Bedivere and Sir William remind Uther that the mages aren't bad people, Mordred had only been a bad leader and now he's gone. After all, it was mage Merlin who gave Uther the sword, and Vortigern himself studied with the mages a few years ago. Uther sees their point and agrees to drop all kinds of aggression against the mages, which Vortigern doesn't approve of.
In the middle of the night, Vortigern tells his men to get ready and takes his wife to a secret underground room in the castle to sacrifice her to the moat hags in exchange for power. As Vortigern's soldiers begin taking over the castle, Uther rushes to wake up his wife and his son Arthur in order to escape through the docks. However a demon knight finds them and kills the couple while the baby escapes on a boat that the river takes all the way to Londinium.
The boat is found by Lucy and her fellow call girls, so little Arthur grows up in a bawdy house thinking he's the bastard son of one of the working ladies while Vortigern takes over the throne. Arthur is bullied by the other kids in the neighborhood, thus he asks kung fu master George to teach him how to fight. Arthur begins developing his street smarts and defends the call girls from aggressive customers while getting involved with the local crime scene and saving the money he makes in a secret spot on the wall.
By the time he becomes an adult, Arthur is the neighborhood's crime boss, yet he still suffers from nightmares that show him the demon knight and the dying woman on the docks. Meanwhile, Vortigern rules over the kingdom cruelly and puts most of his resources into building a giant tower. One morning, he discovers all the water surrounding the castle has receded and this has revealed the location of Excalibur, which is stuck on a rock.
The soldiers are fighting over it, but nobody seems to be able to take it out. Worried about the implications, Vortigern visits the moat hags again to remind them he sacrificed a loved one to get power and that there shouldn't be obstacles. The hags explain that balance is inevitable: if one side of power grows, so must the other.
If he wants to stop the sword, he must find Arthur and finish him. Back to Arthur, he surprises Lucy with some money he took from the last man that hit her. Lucy's impressed because that man had been a viking, and his people can be quite dangerous.
At that moment, Jack and a bunch of royal guards show up at the bawdy house looking for a rebel that attacked the king's barge. Arthur isn't a fan of the king but he doesn't want to put his beloved ladies in danger, so when he finds William hiding in the bawdy house, he hands him over to the guards. Then Jack asks for a private meeting with Arthur and his childhood friends Tristan and Back Lack wanting to know what's going on.
Arthur explains he took money from the vikings because they messed with Lucy and George, and he pretends his team didn't actually use violence. Jack also asks about the graffiti the neighborhood kids have been painting all over town announcing the return of the born king, but Back Lack promises he's already told his son Blue he must stop. Obviously Jack doesn't believe a single word and asks Arthur to be more careful because the vikings are under the protection of the king.
A few days later, Vortigern checks on his guard Mercia about the sword, but unfortunately nobody in the royal army has been able to retrieve it. Mercia is worried because every day the rebels get stronger and the people dislike Vortigern more and more. Vortigern asks Maggie, a lady of the king's court, about the opinion of his people, not knowing Maggie secretly works with the resistance and lies when she tells him the people love their king.
Satisfied by this answer, Vortigern orders Mercia to concentrate on finishing the tower. In the middle of the night, Arthur is woken up by his friends, who warn him the royal guards are coming for him. Arthur tries to escape, but the guards capture him on the streets and take him back to Camelot.
At the harbor, there's a huge line for the test, so Arthur cuts in to avoid more waiting. As soon as Arthur grabs the sword with both hands, the entire kingdom begins shaking and the runes on the sword glow. To everyone's surprise, Arthur takes the sword out of the rock, but the power overwhelms him so much that he passes out.
Later, Arthur wakes up in a cell with Vortigern. The king explains that the sword proves Arthur is Uther's son, but Arthur denies it, saying he's just a call girl's bastard that doesn't want anything. However Vortigern has already sent his men to investigate the bawdy house and found Arthur's hidden money, which means he does have ambition and he's a threat to Vortigern's throne.
Meanwhile in the rebels' hideout, Rubio brings a mage to see Bedivere. This mage has been sent by Merlin to be Arthur's guide, but she needs Bedivere's help to get to the castle. Moments later, all the people in Camelot gather around the castle to see the born king.
Vortigern wants to use this chance to get their respect through fear, and Arthur must play along. If he doesn't surrender the power to Vortigern, then his guards will kill Lucy. When they appear in front of the public, Arthur is given the chance to grab Excalibur and defend himself, but Arthur turns it down to protect his working girls.
Arthur's called a disruptor of piece and he shall die for the sake of the kingdom. When the guards are about to kill Arthur, the mage sends an eagle to stop the guards and makes all the horses and dogs go crazy. The crowd begins running away and the guards concentrate on protecting the king, giving Bedivere and the rebels the chance to rescue Arthur and retrieve Excalibur.
As the working girls take the chance to escape, Arthur runs away with the rebels and the mage while fighting the guards in the middle of the chaos. Two rebels are killed in the process, but this allows the group to get away safely by jumping into a river. Eventually they reach the rebels' hideout, where Arthur is surprised William escaped arrest and is here too.
Bedivere and William explain that Arthur's neighborhood has been torched and he can't go back, they also want to see what he can do with the sword. Arthur pretends he doesn't want to fight before he begins to easily beat some rebels up. When it's William's turn, Arthur grabs Excalibur with both hands and his mind fills with visions, causing him to pass out again.
Hours later, Arthur is woken up by the usual nightmares because the mage is doing a ritual to understand the sword. This time the nightmare shows his younger self handing the sword to Uther, which explains the scars he's always had on his hands. The mage tells him he's resisting the sword and Arthur denies it, saying he isn't the legendary king they want.
However he accepts to give this a chance when the mage points out she can get rid of the nightmares. Afterward, the mage asks Bedivere to take Arthur to the Darklands. Arthur is dropped at an island, where the mage tells him he must reach the tower and touch the altar stone with the sword.
Arthur isn't too worried because he thinks it'll be just like camping, but after he's left alone he learns that the island is full of terrifying creatures. This forces him to use the sword more to defend himself, killing any monster that gets in his way. After a few days of raw survival, Arthur reaches the top of an old tower and puts the sword on the altar stone, which immediately glows and scares all beasts away.
The magic takes over Arthur's mind and shows him the full memory of the night his family died: the demon knight killed the queen, so Uther went after the knight with Excalibur. During the fight, Uther dropped his sword and little Arthur handed it back before his father told him to run. Little Arthur jumped on the boat and let the river guide him as he closed his eyes not to witness his father's death.
Excalibur fell on the back of Uther's body, transforming him into the stone that took the sword down into the water. The demon knight took off his magical veil and revealed his identity to be Vortigern, who planned this whole thing on purpose. After the vision, Arthur passes out and he must be rescued by Bedivere and the mage.
Arthur realizes that the island tower is the same as the one Vortigern is building in Camelot, and the mage explains Vortigern will become as powerful as Mordred when he finishes it, that's why the sword showed up now. Bedivere points out it was Vortigern who started the war against the mages in the first place: he made a deal that he would share power with Mordred if he killed Uther. Mordred accepted, killed the mage king, and took the royal staff to the tower to unlock the dark forces.
However Merlin stole the staff later and made Excalibur with it, using it to destroy the tower. Then he gave Excalibur to the Lady of the Lake, who bound it to Uther's bloodline. When they return to the hideout, Arthur's friends are waiting for him, and they confirm their neighborhood has been burned down by Jack and Mercia while looking for him.
They managed to rescue the working girls and a few friends, but most people ran away. This makes Arthur furious and he decides he'll kill Vortigern as revenge. Afterward, Arthur introduces his friends to the rebels and they begin making a plan.
William wants to meet with the barons from the most important families in the kingdom to ask for their support, but Arthur refuses. He points out those posh lords would never accept him as a leader for being a poor guy with no education, and it's important for the rebels to be the ones in charge. Instead Arthur proposes to lure Vortigern out of the castle and make him come to Londinium by hurting what he cares about the most: the tower.
They begin by sinking the barges carrying the supply of stone and blocking up the river. Then they use William's information on the king's transportation routes to rescue all the kids that are being used as slaves. One of the working ladies knows the man that supplies brandy to Vortigern's favorite palace, so they use it to blow up the premises.
All this forces Vortigern to make a deal with the vikings, even if means accepting shady terms. Before leaving, the vikings show worry over Vortigern losing his power because they also heard rumors about the born king. Desperate to regain control, Vortigern calls for a meeting with all the barons in Londinium, then he goes to the power to begin increasing his powers.
Moments later, Maggie arrives at the hideout and tells everyone about Vortigern's incoming meeting, inspiring the rebels to make a plan to assassinate Vortigern when he visits Londinium. They need a safe building to shoot from, so William proves he can kill anyone with his bow from 175 yards. When the day comes, Arthur watches the king arrive in a carriage and he can't help thinking this is too easy.
It turns out this is all a trap and Vortigern is hiding with Maggie in a boat because he knows she is spying on him and used her to set this up. Arthur can tell the king on the street is an actor, but William decides to shoot anyway in order to kill Mercia. The guards immediately raise the alarm and begin searching the town for the rebels.
Arthur's group tries to hide in the crowd, but the guards find them and a fierce battle begins, during which Back Lack gets hurt. He tells them to go ahead without him, and the group continues to fight through the streets, causing Rubio to get hurt as well. Rubio stays behind too to distract the guards while the group sneaks into George's school because he has a secret tunnel they can use to escape.
However everyone here refuses to be running cowards and they stay to fight. The guards burst in and a huge battle begins during which the mage tries to help by commanding her birds. Her plan doesn't work and she gets captured, which makes Arthur so angry that he grabs Excalibur with both hands and defeats the entire army in a matter of seconds.
This demonstration of power is seen by the neighbors through their windows. Afterward the rebels hide in a safehouse and decide to wait until dark for Back Lack and Blue. On the streets, people are starting to riot because the story of Arthur's powers is spreading, yet Arthur still doesn't want to lead.
Hours later, Blue brings Back Lack to the safehouse, but Back Lack knows he's dying and tells Arthur to leave without him. Arthur drags Blue away right before Vortigern arrives with his guards, who found the safehouse when they followed the kid around. Back Lack pretends to know nothing, but the interrogation is interrupted by Blue, who has come back because he refuses to abandon his father.
Sensing the danger, Blue pretends he's just the cleaner and that he doesn't know Back Lack. Vortigern doesn't believe him and cuts Back Lack's ear as a test, causing Blue to start screaming. Arthur comes back and grabs a guard as a threat, so Vortigern responds by killing Back Lack.
This distraction allows Arthur to grab Blue and run to the boats, now the rebels can finally escape safely. Afterward Vortigern orders his guards to capture or kill any person that is trying to rebel on the streets. When they return to the hideout, Arthur thinks about all the lives that were lost just for him, so he decides to throw the sword into the lake and run away.
The sword is found by the Lady of the Lake and when Arthur falls in the mud, the Lady grabs him to bring him into her lair and show him what Vortigern will do if he isn't stopped. Arthur watches a vision of Camelot completely in ruins under Vortigern's cruelty and finally accepts the sword. Before letting go, the Lady reminds him that Vortigern must be fought in the tower and that Arthur must trust the mage.
When the rebels find Arthur, he's roaring with the power of the glowing sword. Meanwhile Vortigern captures Rubio and hurts him until he gives up information. Moments later, Arthur learns that riots are spreading all over the country in his name.
Arthur announces he's ready to just barge in and take over the castle, so the rebels return to the hideout to get ready. Sadly they find a massacre waiting for them and a royal guard with a message from Vortigern: Arthur must go to the castle before dark, or they'll kill Blue and the mage. The one that goes to the castle is Bedivere because he wants to check it's true the hostages are alive.
Vortigern accepts to exchange Excalibur for the mage and to wait for Arthur until morning. Once the mage reunites with the rebels, she decides it's time to awaken what it's inside Arthur. She summons a snake that bites Arthur's neck and its poison makes him able to see all the magical creatures that hide in nature, the mage also promises this poison will protect him later.
A few hours later, Arthur arrives at the castle while the mage makes her eagle drop a snake inside. Arthur is taken to see the king, but when Vortigern's about to kill his nephew, he notices the snake on a pillar. Vortigern kills the snake and accidentally gets the sword stuck in the stone, so now he can't take it out.
At that moment, the mage sends a giant snake into the castle, and the beast kills all the guards while Arthur retrieves Excalibur. In the streets, the rebels rescue all the prisoners to inspire them to restart the riots. Vortigern wants more power, and the hags remind him there's a price for it.
Seeing no other choice, Vortigern looks for his daughter and kills her as he cries in agony. Then he takes the body to the hags and becomes the demon knight again. Outside, Arthur battles any guard that gets in his way and easily defeats them thanks to the power of his sword.
The guards are so scared that they drop their weapons to let him pass. While the rebels arrive to take over the castle, Arthur enters the tower, and Excalibur reacts to the magic there. Arthur is teleported to the top of the tower where Vortigern is waiting, and a fierce battle begins.
Thanks to his sacrifice, Vortigern is extremely powerful and beats Arthur up in minutes. When Vortigern announces the sword is his now, the poison kicks in and makes Arthur have one last vision. This time Arthur doesn't close his eyes when Uther dies and grabs the sword before it sinks into his father's body.
Uther tells Arthur the sword is his now and Arthur wakes up, finding Excalibur glowing with power. Arthur immediately grabs it and fights Vortigern again with much better control of the sword's powers. Vortigern is hit over and over and Arthur finally manages to kill him as he points out he became who he is thanks to Vortigern sending him to the streets in the first place.
To make his point more ironic, Arthur even kisses Vortigern's hand. Then he activates Excalibur's magic to make the entire tower crumble. The next day, the kingdom says goodbye to all the lives lost during the war.
Afterward, Arthur shows his team the round table he's chosen for their meetings and asks Bedivere to knight his friends. Lastly, William himself knights Arthur, making him the official king. Sometime later, the vikings show up to ask for their part of the deal, threatening with war if they don't get what they want.
Arthur isn't intimidated and reminds them of his power, causing the vikings to kneel and accept him as the new King of England and an ally. At the end of the day, Bedivere crowns Arthur in front of a huge crowd, and Arthur raises his sword as he swears to protect his people.
