The legend of "The Sword in the Stone" receives the modern revisionist treatment in "The Kid Who Would Be King," an engaging if somewhat overextended coming-of-age fantasy written and directed by Joe Cornish (2011's "Attack the Block"). Louis Ashbourne Serkis (2016's "Alice Through the Looking Glass") is a warm, sympathetic natural as Alex Elliot, a 12-year-old Londoner who pulls King Arthur's ancient sword of Excalibur from a construction site and, to his amazement, discovers he is the chosen one to stop the impending takeover of Arthur's resurrected, power-hungry half-sister Morgana (Rebecca Ferguson). As Alex sets about forming his personal Round Table of knights—best friend Bedders (Dean Chaumoo) and bullies Lance (Tom Taylor) and Kaye (Rihanna Dorris)—he is guided along on his perilous journey by shape-shifting wizard Merlin (Patrick Stewart as the elder Merlin, a scene-stealing Angus Imrie in teenage form).

"The Kid Who Would Be King" is amiable through and through, but the longer the narrative unspools the more it begins to lack a palpable threat. Yes, the very planet is meant to be at stake, but the lives of the characters—whether they are being chased by fiery skeletons on horseback or battling Morgana in giant dragon form—never appear to be in real danger. It's also something of a crime to cast a talent of Rebecca Ferguson's (2018's "Mission: Impossible – Fallout") caliber as villainess Morgana and then give her next to nothing to do but slink around an underground lair.

What works so well in the film, then, is young Alex's personal journey. He isn't simply tasked with saving the world, but also with something equally tough: growing up. His mission is a compelling one, perhaps less so for its more action-oriented elements than the harder truths he must learn about his loving mum (Denise Gough), his absentee father, and antagonistic classmates Lucas and Kaye, capable of being better versions of the people they present themselves to be. "The Kid Who Would Be King" goes on a bit too long—the entire third act could have been excised and not been missed—but this is nonetheless an inviting, often wise family film, boasted by wonderful protagonist Alex at its center.