Alexander "Alex" Elliot, portrayed by Louis Ashbourne Serkis, is a twelve-year-old boy grappling with the challenges of a new school term while trying to fit in. Everything intensifies when his best friend, Dean Chaumoo, is bullied by older students, namely Tom Taylor and Rhianna Dorris. Alex comes to Bedders' defense, leading to an unfortunate confrontation with Lance. As a consequence, all three boys find themselves in detention, where the headmistress urges Alex to embrace his potential.
In the shadows, Lance and Kaye scheme to torment Alex further. That very night, they pursue him home, but Alex cleverly hides in a nearby construction site, stumbling upon a mystical sword. Excited by his find, he soon discovers that it’s none other than Excalibur, the famed sword of King Arthur. With a glint of mischief, he playfully "knights" Bedders, marking the beginning of an extraordinary adventure.
Meanwhile, the malevolent sorceress Rebecca Ferguson, portraying Morgana, stirs from her underground slumber, sending her dark minions in search of Excalibur. The next day, a mysterious new student, Angus Imrie, emerges from Stonehenge and introduces himself as Merlin, the wizard who can age in reverse and navigate between his various Arthurian identities. Uninterested in ancient legends, Alex initially wishes to return the sword.
That evening, Merlin rescues Alex from a demon and reveals that he has only four days to thwart Morgana’s plans, or she will subjugate all of England. Her minions, visible only to Alex and those he has knighted, have a window of power only at night, and a total solar eclipse approaches that will allow Morgana to break free. Alex discovers that these events mirror a storybook gifted to him by his estranged father. Realizing he descends from King Arthur, Alex decides to knight both Lance and Kaye, forming a modern-day Round Table to stand against the encroaching darkness.
Journeying to Tintagel, where he hopes to find answers about his father, Alex trains under Merlin’s watchful eye. However, treachery unfolds when Lance betrays him, seizing Excalibur. In the ensuing chaos, the sword is shattered during a struggle in a marsh, but the Lady of the Lake intervenes, restoring it. Reinvigorated, Alex and his friends devise cunning strategies to navigate hordes of demons, culminating in their arrival at Tintagel.
Here, Alex meets his aunt, who reveals a painful truth about his father’s past as an alcoholic and the lies surrounding his family history—pain that deepens his resentment toward his mother, played by Denise Gough. Nevertheless, Merlin imparts wisdom, teaching Alex that true worthiness does not stem from lineage, but from personal merit.
Determined, Alex uses the storybook to uncover the entrance to Morgana’s lair. Confronting her in a battle of valor, Alex strikes Morgana down, believing her vanquished. Yet, during the eclipse, Merlin warns that she was merely wounded. In a moment of vulnerability, Alex confesses everything to his mother, who in turn reveals her own truths, including the reasons behind her silence.
In a shocking twist, Alex magically summons the Lady of the Lake into his bathtub, reclaiming Excalibur. At school, Merlin casts a spell to enchant the faculty, and Alex knights his fellow students. As Morgana emerges in her formidable draconian form with her army, the children combine their medieval tactics with modern technologies in a valiant fight. Eventually, they find themselves retreating to the rooftop, where Merlin crafts a spell to pull Morgana from this realm. In a climactic moment, Alex decapitates her as she fades away, dispelling her monstrous army.
As the dust settles, Alex, Bedders, Lance, and Kaye bid an emotional farewell to Merlin, who inspires them to step into their roles as leaders.
