Plot summary

Alexander "Alex" Elliot (Louis Ashbourne Serkis) is a twelve-year-old boy who is starting a new term at school while struggling to adapt to his new surroundings. When his best friend Bedders (Dean Chaumoo) is bullied by older students named Lance (Tom Taylor) and Kaye (Rhianna Dorris), Alex comes to his aid but ends up in a fight with Lance. Alex, Lance, and Kaye are given detention by the headmistress while she tries to encourage Alex to live up to himself.Meanwhile, Lance and Kaye plot to harm Alex further. That night, the duo chase Alex as he is on his way home but Alex hides in a nearby construction site, where he finds a mysterious sword and removes it from its position. Taking the sword with him, Alex shows it to Bedders, and they discover that its markings identify it as Excalibur, the sword of King Arthur. Alex then playfully "knights" Bedders.Elsewhere, the wicked sorceress Morgana (Rebecca Ferguson) awakens underground and sends her minions after Excalibur. The next day, a teenager appears from inside Stonehenge and presents himself at Alex's school as a new student. The boy reveals himself to Alex as the wizard Merlin (Angus Imrie), capable of aging backwards, but is also able to shift between his Arthurian counterparts. Alex plans to return the sword, wanting nothing to do with ancient myths.That night, Merlin saves Alex from a demon and explains that he has four days to destroy Morgana, or she will enslave all of England. Morgana's demons can appear only at night and can only be seen by Alex and those he has knighted, but an upcoming total solar eclipse will enable her to emerge fully into the world. Alex realizes that these events parallel an inscribed storybook his estranged father once gave him. Alex concludes he descends from Arthur through his father and later recruits and knights Lance and Kaye, who fight beside Alex and Bedders, defeating three demons. Alex declares them a new Round Table. Merlin soon tasks Alex to find the entrance of Morgana's prison.Believing his father is the key, Alex leads the group to Tintagel where he last saw his father. En route, Merlin trains them in swordsmanship. But when Morgana infiltrates the lesson, Lance betrays Alex and takes the sword for himself. Merlin barely saves them, and Excalibur is broken when Alex and Lance come to blows in a marsh. Alex calls upon the Lady of the Lake who appears and restores the sword.Rededicating themselves to the quest, the four overcome a horde of demons by luring them over a cliff and arrive at Tintagel. Alex meets his aunt Sophie (Genevieve O'Reilly) who tells him that his father was revealed to be an alcoholic, a fundamentally bad person who abandoned Alex's family, and that it was secretly his mother (Denise Gough) who inscribed the book. Alex is enraged to discover that not only has he come a long way for nothing but the fact that his mother lied to him about the book, and he starts to hate her for that. Merlin tells Alex that Excalibur is not handed down by birthright, but by individual merit.Alex and his friends arm themselves and Alex uses the storybook to locate the entrance to the Underworld (it was Morgag. Alex challenges Morgana who takes on a monstrous form and breathes fire, but Alex strikes her down and the children escape. Believing Morgana is dead, Alex returns Excalibur to the Lady of the Lake.On the day of the eclipse, Merlin informs Alex that Morgana was merely wounded, and Alex realizes that he violated the Chivalric Code by lying to his mother. In desperation, Alex tells her everything that has happened. His mother reveals her own lie that she never told him the truth about his father or the book, because if he had known the truth to begin with, it would've hurt him. Eventually, he stuns her by summoning the Lady of the Lake into the bathtub, where he regains Excalibur.At the school, Merlin enchants the faculty and Alex knights the entire student body. During the eclipse, Morgana appears with her entire army and now takes on a huge, semi-draconian form. The children fight back, using strategies combining medieval warfare with modern technology, but ultimately retreat to the rooftop. Merlin casts a magic spell to pull Morgana from the world and Alex decapitates her as she vanishes, dispelling all the demons. Alex, Bedders, Lance, and Kaye bid farewell to Merlin, who encourages them to become leaders.