Joe Cornish’s first movie “Attack the Block” was about a pretty wild bunch of teens doing battle with space aliens. His second is about youngsters too, this time fighting a malevolent sorceress, but they’re a very mild group by comparison. As the title indicates, “The Kid Who Would Be King” is a modern riff on Arthurian tales (there are reminiscences sprinkled throughout, some mere throwaways like a street named “Malory Lane”), but though nicely made in widescreen on attractive locations, it comes across like a made-for-Nickolodeon-quality kidflick that’s unaccountably escaped into theatres.

The boy of the title is Alex (Louis Ashbourne Serkis, son of motion-capture star Andy), an utterly ordinary student at Dungate School who nonetheless tries to protect his best friend, nerdy Bedders (Dean Chaumoo) from campus bullies Kaye (Rhianna Dorris) and Lance (Tom Taylor). The nasty duo chases Alex into a construction site where he finds the Sword in the Stone—King Arthur’s Excalibur—and pulls it from its place, identifying him as the Chosen One.

His selection—not, as will be explained in a typically modern gloss, because he is of Arthur’s line but because everyone, especially a young person, is potentially “special”—is necessitated by the danger posed by the evil Morgana (Rebecca Ferguson), Arthur’s nemesis, who has been trapped underground for centuries and lusts after the sword. Oozing branch-like tendrils and able to summon up demonic horsemen to track down her enemies, she resumes her quest to subjugate England and the world, which—as some opening news reports tell us—is in dire international turmoil.

Alex has two sources of information he can consult for information on his new role. One is a children’s book on Arthur left him, her thinks, by his absent father (he’s been raised by a single mom played by Denise Gough); it suggests to him that he must be of the Arthurian line, leading him to undertake a quest to the Isle of Tintagel, Arthur’s home, where he expects to find his dad. The other is the sudden appearance of Merlin in the form of an eccentric young man (Angus Imrie) who shows up at Dungate calling himself Mertin, though he occasionally changes form to become either a CGI owl or the elderly wizard (Patrick Stewart) himself, the transformation effected by a sneeze.

Nor will Alex make his journey alone. He will of course be accompanied by Bedders, but also by the initially reluctant Kaye and Lance; he knights them all, and together they will undergo training at Marlin’s hand, evade Morgana’s walking trees (shades of “H.R. Pufnstuf”) and fire-eyed warriors, and eventually make their way not only to Tintagel but Stonehenge (a transport device, here) and Glastonbury, the gateway to Morgana’s realm, where in their new knightly garb they will face off against her.

So far, so good, and if the picture ended there at roughly the ninety-minute point, it might just pass muster. Serkis, quite honestly, doesn’t make a particularly compelling protagonist—he could switch places with nebbishy Chaumoo and you’d barely notice—but his utter lack of charisma is of a piece with Cornish’s argument that ordinary people are the ones who will make the difference (especially if they’re kids, as Stewart’s Merlin tells us in an especially pandering passage); and his three comrades-in-arms—Chaumoo, Dorris and Taylor—are all okay, if equally unexceptional. More engaging is Imrie as the oddball teen Mertin, whose method of spell-casting through elaborate hand gestures and finger-snapping is overused but still more amusing than much else that happens onscreen. Stewart delivers his customary stentorian turn, but Ferguson is pretty much lost in makeup. The owl is fine (the CGI horsemen less so), and cinematographer Bill Pope gives us some lovely widescreen shots of the English countryside (and of Tintagel Island) in the course of the kid’s quest.

But Cornish isn’t satisfied with this. He adds another half-hour back at Dungate, where Alex must enlist the entire student body to join his ragtag army against Morgana, who wasn’t defeated for good, after all, and her apparently inexhaustible hordes of nightmarish horsemen. This protracted coda is chaotically staged and, quite frankly, a drag, with its entirely gratuitous and poorly-rendered effects (Morgana even turns into a dragon). As it lumbers on, the movie sinks faster than Excalibur being taken beneath the waves by the arm of the Lady in the Lake (uncredited).

One shouldn’t be too hard on “The Kid Who Would Be King”—it’s totally bland but innocuous, a perfectly ordinary family film trying desperately to prove it’s special. Unlike the movie’s pint-sized hero, it fails.