“Sometimes,” says self-made musical impresario Jan Lewan (Jack Black) in The Polka King, “to make money you have to spend money.” The problem with Jan is he didn’t spend money sometimes, he did it every time. Every get-rich quick scheme he ever hatched up in his Pennsylvania gift shop required more investment than potential profit. He sold his polka fans on a “premium Pope package,” a lavish vacation to Rome that included a private audience with the Pope. But Jan had no way to deliver on his promise, so he just showed up at the Vatican with a briefcase full of cash. He got his audience, and delighted his tour group. But his “premium” vacation surely cost him more than he made.

Lewan is a real guy; The Polka King, by writing/directing team Maya Forbes and Wallace Wolodarsky, is based on a documentary about the real Lewan, The Man Who Would Be Polka King. The feature version takes many outlandish twists and turns, most too absurd to be true. And yet they’re all in the trailer for the documentary, right down to some of the lines of dialogue. This is one of those stories when reality was stranger, and more entertaining, than fiction.

Lewan became a local legend in Pennsylvania for his wild polka concerts complete with dancing, a full band, and even a woman dressed like a bear. (Vanessa Bayer plays the bear, which technically means she’s a “Vanessa Bear,” which technically means The Polka King is incredible.) ,

His crowds are enthusiastic but not gigantic, so how can he afford to pay all the members of his ever-expanding orchestra? He can’t, and as the film begins Jan is in danger of losing his partner Mickey (Jason Schwartzman) to a job at Radio Shack. Desperate and broke, Jan cooks up a plan to convince his customers to “invest” in his company. Give me your money, he tells people in his thick Polish accent, and he guarantees 12 percent return on investment; four times what the banks promise. How can he afford 12 percent return on investment from a floundering polka band and an empty storefront in a strip mall? He can’t. Perhaps you sense a pattern forming.

Jan constantly needs new investors to pay off the old ones, and pretty soon he’s the kingpin of a full-fledged Ponzi scheme. There are red flags everywhere, and even an investigation from the federal government (in the form of a very understated and very funny JB Smoove), but Jan kept “American Polka Inc.” running for years before an absurd scandal involving his wife Marla (Jenny Slate) and a beauty pageant finally brought his secrets to light. Still, even with all the obvious holes in his business plan (like the part where he never actually makes any money), it’s easy to see why people kept investing in Jan Lewan. A tireless worker and an endless charmer, Jan was everyone’s friend. He worked two, three, or four jobs at a time. When his friends struggled, he paid their bills. He was always moving towards his next big idea, and he had the balls to reach for his goals. And even though he was terrible with money, he actually achieved a lot of them. (Hell, he was even nominated for a Grammy.) His victims kept going along because they wanted to believe he was real. It would have been great if it was.

A self-made man, a huckster, and an immigrant who believed in the American dream so much it nearly destroyed him, Jan Lewan is the perfect movie character for our times. And he’s a perfect character for Jack Black, who gets to exercise his ample talents as a musician in addition to his natural comic timing. Plus, no matter how big Black goes (and he goes big), it never seems too big for Jan Lewan. This guy is larger than life. The only person who saw through his facade was his grumpy mother-in-law (Jacki Weaver, who steals so many scenes with her fiery tirades she’s technically qualifies the second-biggest thief in The Polka King).

Forbes and Wolodarsky don’t try to do too much with The Polka King; with a story this good, you don’t have to. They assembled a terrific cast, and unleashed them on an incredible tale of ambition, greed, and dancing bears. The very likable results have all the makings of an arthouse hit. All I want to know now is: What’s the real Jan’s plan to get rich off this movie?