If you’ve seen the popular YouTube clip of Paul Potts’ audition for the television show “Britain’s Got Talent,” you may have wondered, like many others, “Where did this schlubby guy with the big, beautiful voice come from?” You can see that question in the gobsmacked expressions of the talent show’s judges – Piers Morgan, Amanda Holden and Simon Cowell – who clearly expected little, if anything, from Potts, a paunchy Welsh mobile-phone salesman with bad teeth and an obvious case of stage fright (who ultimately went on to win the competition and 100,000 pounds).

It’s a question that’s answered in “One Chance,” an amiably crowd-pleasing dramatization of Potts’ rocky road to fame and fortune that’s made more engaging than similar fact-based fairy tales by James Corden’s outsize performance as Paul. Playing the opera-loving protagonist with a mix of self-effacing charm and innate belief in his singing ability, the young English actor (“Gavin and Stacey”) brings enormous charisma to a role that would be all too easy to turn into a caricature. As his love interest Julz, Welsh TV veteran Alexandra Roach is equally beguiling. And the singing, dubbed by the real Potts, is a delight, even for non-opera fans.

Part rom-com and part Cinderfella story, the film – for which Cowell was a producer – breaks no new dramatic ground. Culminating with that 2007 “BGT” audition, the movie treads a well-worn path toward its ultimate climax, beginning with our hero’s difficult childhood in a small, steel-mill town, where Paul was bullied for his unorthodox musical tastes. Yet despite the story’s familiarity, its star manages to turn its many tropes into a winning formula.

An early run-in with a metal post, while fleeing schoolmates, accounts for Paul’s ruined smile (so evident in that YouTube clip). Never repaired – or at least not until after winning the TV show – Paul’s cracked teeth are a significant, if peripheral, part of the character’s goofy appeal. It’s really Paul’s wry, self-deprecating sense of humor, forged and polished through numerous encounters with a disapproving father (Colm Meaney) and a nutty store manager (Mackenzie Crook) that better explain why we like this guy. As much of a sad-sack Everyman he is, it’s easy to see why Julz falls for him, especially when he opens his mouth. Whether singing or stammering out an apology, he’s a sweetheart.

Taking its title from Potts’ best-selling debut album, “One Chance” is billed, in Corden’s voice-over narration, as the story of the “opera” of Potts’ life. Yet despite the humor and drama of that life – a long-shot success story heightened by several medical emergencies, including a ruptured eardrum, thyroid cancer and an automobile accident, as well as bombing in front of Luciano Pavarotti while taking voice lessons in Venice – the peaks and valleys of Paul’s narrative are relatively flat.

There’s never really terribly much at stake here, except the hopes and dreams of one chubby nerd who knows how to nail Puccini’s “Nessun dorma.” And there may be a little of that guy in all of us. Three stars. Rated PG-13.