Young Alexander Elliot finds the legendary sword Excalibur and pulls it from a stone. The wizard Merlin finds him and warns him that he is now destined to fight the evil sorceress Morgana, as she is set to return when the moon is eclipsed, and she will bring a dark army, the Mortes Milles back to reign over Britain, now that it is in a state of disarray. Alex rounds up his best friend Bedders and bullies Lance and Kaye to become knights and defeat Morgana.
The friends travel with Merlin to the island of Tintagel to find the entrance to the underworld to fight Morgana, while Alex also tries to find his long lost father. He learns from his aunt Sophie that his father was an alcoholic who didn’t care for him or his mother. This disheartens Alex until his friends motivate him to keep pushing through. They come across Morgana’s lair and fight her, with Alex seemingly killing her.
It turns out Morgana is still alive because Alex wasn’t true to his loved ones. He goes to his mother and apologizes for being angry with her for lying and saying his father was a hero. He proves to her that Excalibur is real, and he goes with his friends to rally the students to become an army and fight Morgana. They do so and prepare as she and her army rise from the underworld. Alex fights her and decapitates her, ending her once and for all.
Alex returns Excalibur to the Lady of the Lake until he is needed again. For now, he and his friends plan to be the good that the world needs.
Many years ago in Britain, King Arthur pulled a magical sword from a stone and converted his enemies into his faithful knights of the round table. Aided by the wizard Merlin (Patrick Stewart), Arthur and his knights defeated the evil sorceress Morgana LeFay (Rebecca Ferguson) and trapped her in the center of the Earth. Arthur served as an honorable and chivalrous king and ushered in an age of peace. But as world leaders have become less honorable and chivalrous, and humanity becomes more divided, Morgana awakens and begins breaking free of her bonds.
Alex (Louis Ashbourne Serkis) is a geeky, undersized high school freshman who is constantly picked on by school bullies Lance (Tom Taylor) and Kay (Rhianna Dorris). One night, the bullies chase him to a construction site. He comes upon a sword embedded in a rock and pulls it out before fleeing home. Alex’s best friend and fellow geek Bedders (Dean Chaumoo) comes over and jokes that the sword must be King Arthur’s sword. After Bedders leaves, Alex pulls out an old book his absent dad left him about King Arthur that is inscribed “To Alex, my once and future king.”
The removal of the sword awakens Merlin, who seeks out the new wielder of the sword. He disguises himself as a teenager (Angus Imrie) and tries to convince Alex he is chosen to lead the impending battle against Morgana. Alex ignores Merlin, but is attacked that night by an undead soldier loyal to Morgana. Now believing Merlin, he seeks the wizard out. Merlin tells him that only individuals knighted by Alex can see Morgana and her forces and help fight her. Morgana can only attack at night, and Merlin can only aid him in daylight (his powers and lifeforce are diminished at night). Morgana’s powers will be strongest during an upcoming solar eclipse and Alex must try and defeat her before then.
Although Alex initially refuses to fight, he is inspired by his dad’s inscription. He knights Bedders and, later, Lance and Kay (who think it is a lame joke, but want to fight with the swords) as they are the strongest kids he knows. Eventually they all agree to join Merlin on a quest to Morgana’s castle (conveniently located where Alex believes his dad lives). Alex’s mom forbids the journey, but he runs away from her. Along the way, Merlin teaches the kids how to fight and tutors them in the ways of honor and chivalry. Lance and Kay initially are selfish and bullying, but eventually come to respect Alex and Bedders . Although Bedders is physically weak, he proves adept at magical spells. They arrive in the town where Morgana’s castle is as a united group of warriors who survived several attacks by Morgana’s undead army.
Before they go to confront Morgana, Alex goes to visit his dad. His long lost aunt (his dad’s sister) answers the door and explains that Alex’s dad was a drunk who abandoned him. When Alex shows his aunt the inscription in his book, the aunt explains that the inscription is in his mother’s handwriting (and his mom said it was his dad’s inscription to hide the truth and make Alex feel better). Alex threatens to abandon the mission, but is convinced by his friends and Merlin to stay the course. The four warriors go into Morgana’s castle. She turns into a horrible dragon creature and fights them. The kids wound Morgana, but flee before ensuring her demise.
The warriors return home, thinking they were successful. Alex reconciles with his mother and they promise to love and be truthful to one another. The kids eventually realize Morgana still lives and that the eclipse is in mere hours. They go to their school and eventually convince all their classmates to be knighted and join Alex in battle. Bedders and Merlin use magic to duplicate armor and swords for the students and Kay and Lance train them how to fight. During the eclipse, Merlin can also aid them as it is neither night nor day.
When the eclipse occurs, Morgana and her undead army attack the school. The students’ traps and training overcome her army and eventually, Alex lands a killing blow on Morgana. The students celebrate as Alex, Bedders, Kay, and Lance accompany Merlin to a lake to return Excalibur. Merlin says in his centuries of life, he never witnessed a braver or better fought battle. The kids express their concern that the world is still a fractured place and Merlin responds that the younger generation has proven itself and will become nobler and better leaders for the world.
The film opens with the wizard Merlin (Patrick Stewart) telling the tale of King Arthur. After the death of King Uther Pendragon, Britain fell into war with many wanting to rule over the land. Merlin crafted Excalibur to see who would be worthy of becoming king. Arthur pulled the sword from the stone, but his half-sister Morgana Le Fay (Rebecca Ferguson) wanted power to herself and to cast a dark magic over Britain. Arthur and his knights fought against Morgana and cast her into the underworld, where she vowed to return when the world was at its darkest point.In the present day, 12-year-old Alexander Elliot (Louis Ashborne Serkis) lives with his mother (Denise Gough) and attends a private school. On the bus ride to school, he gets a text from his friend Bedders (Dean Chaumoo), who is getting harassed by bullies Lance (Tom Taylor) and Kaye (Rhianna Doris). Alex stands up to them and orders them to leave Bedders alone. Lance tries to intimidate Alex and stop him from trying to be a hero. As he walks away, Alex runs after him and pounces on his back, beating on him in front of the other kids. Mrs. Elliot later finds a bruise on Alex’s face from the fight. Alex wants to make right the wrongs of the world, even though it gets him into trouble, as he feels his estranged father would want him to do so. His mom hasn’t told him much about his dad but has made him seem like some kind of hero who still cared for Alex.
The next day after school, Lance and Kaye chase after Alex to finish the fight. Alex runs toward a building site and is cornered near an edge by the bullies until he falls backward and is knocked unconscious. Lance and Kaye run away in fear. Alex comes to and finds a sword stuck in a stone. He manages to pull it out and wield it. Deep below in the underworld, Morgana awakens and senses that the sword has been found. She creates vines that will seek out the one that now wields the sword.
Alex calls Bedders over to check out the sword. After translating what was written on the sword, they realize it is Excalibur, as Alex keeps a book given to him by his father on King Arthur and the Knights of the Round Table and recognizes the world Tintagel from the book as being an island where Excalibur was created. Alex decides to take the sword and knight Bedders.
Merlin senses that the sword has been found, and so he walks out into town looking like a teenager (now played by Angus Imrie). He makes it to Alex’s school to pretend to be a schoolboy and get closer to Alex. Merlin learns from a teacher that an eclipse is coming within three days time, which causes him to panic. He later bursts into the cafeteria to tell Alex he knows he pulled Excalibur out and that Morgana will rise again when the eclipse occurs, which just freaks Alex and Bedders out.
That night, Alex finds a skeletal minion of Morgana’s coming to attack him in his house, and his mom is nowhere to be found. Alex rushes toward Excalibur, but just as the minion tries to strike, it is destroyed by Merlin. However, being in the mortal realm after sundown is dangerous for him, and he quickly retreats in his owl form.
Alex and Bedders find Merlin working at a fried chicken shop since the ingredients in the nuggets, soda, and ice cream from there contain elements that provide him sustenance (beetle blood, beaver urine, and ground animal bones). He tells the boys about Morgana and how she plans to come back now that the country has fallen into chaos and disarray. Alex doesn’t want to take on the responsibility of saving everyone until Merlin turns into his old self and shows the boys a vision of what everything will look like if they do not find Morgana in the underworld and fight her there before she is free and unleashes her army, the Mortes Milles. Alex is freaked out again but now knows it is his duty to save the day. He tells Bedders that they need to go to Tintagel to find his father, since he thinks that like Arthur, who didn’t know his father, Alex is meant to find him and have him help them stop Morgana.
The boys go by the building site again and come across Lance and Kaye. Alex puts the sword back in the stone and challenges the bullies to pull it out. After they fail, he pulls it out again. He convinces the two to kneel so they can be knighted, and although they think it’s a dumb idea, they do so.The Mortes Milles then show up to attack. Lance tries to take the sword away from Alex and fight them off himself, but it doesn’t work. Alex helps him up as the four run away and find the streets empty, as everyone who hasn’t been knighted will vanish when the Mortes Milles appear. They steal a car and manage to destroy the minions, but this brings everyone else back and almost causes a traffic accident in the chaos. The four go back to Alex’s house where he manages to convince everyone to join him in a quest to Tintagel to find the entrance to the underworld and fight Morgana. However, Morgana has her eye on them and senses the weaknesses in each of them, hoping to use these to her advantages.
Alex leaves a note for his mom, who had previously tried to get him to stop talking about Excalibur as if it were real, and now she thinks he has run away. He joins Bedders, Lance, Kaye, and Merlin as they take a bus to Stonehenge where they will come closer to finding the entrance to the underworld. After passing through, Merlin has the four train to be better fighters. They fight against dark roots controlled by Morgana. She uses her magic to manipulate Lance into taking Excalibur for himself. The roots grow stronger and pull the kids deeper into the earth until they call out for Merlin, who returns and sends the roots back underground. The others chide Lance for what he did, even though he thinks he deserves the sword more. Alex and Lance then fight, resulting in Lance breaking Excalibur. Alex tells Lance off for ruining everything and just being a cynical jerk. Kaye apologizes on their behalf but adds that they are bullies because the world is already rotten. As they are passing through a swampy area, Alex drops the broken sword into the water, which is then returned to him after he realizes he can summon the fabled Lady of the Lake. It works, and she gives Alex a repaired Excalibur.
The friends set up camp for the night. Alex goes off on his own to Tintagel to find his father. He comes across a house off an address he got from his mom. There, he finds his Aunt Sophie (Genevieve O’Kelley). Alex asks for him, believing that he was a hero like his mom made him out to be, and that he was fighting demons as she described to him. Sophie then tells Alex the truth about his father. His demon was alcoholism, and he left him and his mom because he didn’t care about them. Mrs. Elliot tried to visit him with Alex at one point to see if he changed, but he hadn’t. The book wasn’t even from him, it was from his mother. Alex is heartbroken to find out what kind of man his father was, and he returns to Bedders feeling hopeless. Bedders then uses a metal-duplicating trick he learned from Merlin in a way to cheer him up and convince him that he is not hopeless to face Morgana. Lance, Kaye, and Merlin also show up to give Alex a confidence boost.
The heroes ride horses until they come across an area that leads to the underworld. Down there, Alex comes face-to-face with Morgana. She sends her minions to attack his friends as she transforms into a monstrous flying creature who tries to kill Alex. He wields Excalibur and then stabs Morgana in the chest, seemingly killing her. The place starts to collapse on itself, but the heroes get out before they are trapped. Alex throws the sword to the Lady of the Lake and returns home, thinking he is victorious.
The next morning, Alex awakens to find his house covered in roots, meaning Morgana is alive, and the eclipse is happening that night. He realizes that although he thought he followed the code needed to defeat Morgana, he wasn’t true to his loved ones. Alex goes to his mother and apologizes for how he acted with her and being angry at her for lying about his father. He then goes to the bathtub to prove to her that Excalibur is real by having the Lady of the Lake return him the sword.
With help from his friends, Alex convinces the headmistress to round up all the students together to help him fight. They laugh at first, thinking he is just being a nerd, until Merlin proves who he is with his magic, and the students are convinced to join after the headmistress tells them this will be instead of lessons that day. Bedders duplicates armor and weapons for the students to use.
The eclipse comes, and so does Morgana and the Mortes Milles. The students gather and take out the skeleton demons, but even more start coming out to fight. Morgana finds Merlin and weakens him. Alex finds him and brings him liquified nuggets, soda, and ice cream to revive him. It works, and Merlin helps the students fight back. Alex gets close enough to get on top of Morgana and strike her with the sword, decapitating her. Her monstrous body is pulled into the underworld as the rest of the demons are vanquished. Everything is back to normal.
Merlin says farewell to Alex and his friends, as there will come another time when they are needed again. For now, Alex has new friends and a new sense of purpose to be the good in a dark world. With that, Alex throws Excalibur back toward the Lady of the Lake.