Athletes aren't known to be great actors, but "Uncle Drew" inspires such killer stuff from its cast of NBA legends that even Shaq delivers.

The best film ever adapted from a series of soft drink commercials (tough luck, “Space Jam”), Charles Stone III’s “Uncle Drew” is such a well-acted, warm-hearted basketball comedy that you’re liable to forget about its corporate origins. To be honest, it’s hard to think about such things when you’re struggling to make sense out of Reggie Miller — the ex-Pacer whose three-point daggers taught a generation of young Knicks fans what it means to hate another human being — being an immensely likable actor. And a good one.

Of course, you’re also liable to forget that Reggie Miller is even onscreen in the first place. Like most of the NBA legends who star in this movie, Miller is playing an old-timer several decades his senior, and the Hall of Famer is rendered virtually unrecognizable by layers and layers of Oscar-worthy makeup. To clarify: All of the NBA stars in “Uncle Drew” are caked in all sorts of sinewy latex, and most of them are rendered virtually unrecognizable. At the end of the day, there’s just no way of disguising Shaq.

The basic premise — a throwback to the glory days of tournament movies — is as simple as it is effective. Dax (“Get Out” star Lil Rel Howery, a likable straight man in his first leading role) is a dude with skinny calves and a head that’s almost as big as his hoop dreams: He wants to coach a streetball team to victory at Harlem’s annual Rucker Tournament, winning the $100,000 prize and getting sweet revenge over the rival who bullied him back when they were both players (Nick Kroll is delightfully noxious as Mookie, aka “the ghost of white boy past”). The trouble is that Dax’s best player has just defected to Mookie’s team, and our hero’s gold-digging girlfriend (Tiffany Haddish, doing a lot with a little) is just as loyal.

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Dax’s only hope? Recruit streetball legend Uncle Drew (current Boston Celtic Kyrie Irving), a player so good he used to dunk on people with the rock in one hand and a ham sandwich in the other. The good news is that Drew can still flex on the youths when he feels like it, and Dax inspires the old man to feel like it. The bad news is that Drew has a few conditions. The first is severe arthritis. The second and more serious of his conditions is that he gets to pick the other players. Dax reluctantly assents, and so they hop into Drew’s musty van (a sex palace on wheels, complete with shag carpet and a “boom-boom room” in the back) and head south on a road trip to recruit some of the old-timer’s former teammates.

Even before Dax and his new crotchety new best friend make it out of Manhattan and start their “Blues Brothers” basketball tour, it’s already clear that “Uncle Drew” is no mere cash grab. Irving’s performance alone endows the project with more integrity than you could find in all of “The Emoji Movie,” or in the “Cavemen” sitcom that was mercy-killed by ABC after they bet on the idea that Americans wanted their Geico commercials to be 22 minutes long.

Irving, who wrote and directed the original “Uncle Drew” shorts, exhibits a deep understanding of who the character is and what continues to haunt him. Drew is a living myth, an urban legend, a wrinkled man who had to settle for rumors because he never became a champion. He’s also a curmudgeon who refers to everyone shy of 50 as “youngblood,” and bemoans how basketball is a symphony that all of today’s punk kids play like a solo. Sure, he can run a game by himself, but there’s no music in that — he’d rather be the sixth man on the Warriors than carry Lebron’s load for the Cavs.

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In other words, Drew is nothing without his teammates, and Irving fittingly spends most of the movie setting up his cast mates for easy slam dunks; he’s got strong comic timing, but his main objective is to offer an emotional anchor for everything around him. And Irving does just that. He’s sometimes funny, but he’s always good. Lebron didn’t even give this convincing a performance in “Trainwreck,” and he was playing himself.

The rest of the cast rises to Irving’s level. Chris Webber completely disappears into his role as Preacher, a former power forward who’s traded in basketball for baptisms (his pissed off Terminator of a wife, played by WNBA great Lisa Leslie, drives some empty comedy until she’s asked to come up big in the third act). The historically sadistic Reggie Miller owns some low-key laughs as a blind shooter named Lights, and Nate Robinson speaks volumes with his silence as a wheelchair-bound point guard called Boots. Each new player Uncle Drew recruits is more debilitated than the last, to the point where you start to suspect the guy is going to round out the team with a corpse. Instead, he finds Shaq.

Rocking a gray (Shaq)Fu Manchu and running a martial-arts dojo, Big Fella is Drew’s most embittered former pal, and the glue that Dax needs to hold his squad together. He’s more than 7” tall of pure resentment, a man so angry that his sideburns peel off his face out of rage (it’s hard to say if Shaq’s resistance to makeup is part of the joke, but it’s always funny all the same).

In a movie that hones in on themes of redemption, and the courage required to confront your fears (Drew is known to quote Wayne Gretzky’s mantra that “you miss 100% of the shots you don’t take”), Shaq’s casting assumes a meta-textual element. Here’s one of the most successful celebrity businessmen in America returning to the big screen — one of the few arenas where he’s failed in the past. “You can’t always play your way out of your problems,” someone advises, but that doesn’t mean you can shrink away from them.

And so Shaq, who’s acting career peaked with “Blue Chips” before imploding when the athlete decided he could be a leading man, gets another chance to show off his chops. And he does. His performance as Big Fella isn’t going to fill the hole that Daniel Day-Lewis’ retirement left behind, but it showcases much sharper comic gifts than “Grown Ups 2” ever saw in him. Not everyone can turn an asscheek reveal into art, but Shaq knows what he’s doing. Shaq earns his redemption, and the whole film resonates around that.

As director, Charles Stone III deserves credit for letting the movie be carried by that emotional undertow. There are some laughs here, but nothing gut-busting — Stone doesn’t force the issue. He’s fine with a movie that’s more of a friendly game than a blowout, a movie that scores most of its points on lay-ups, and that’s more than pleasant enough for the right price.

At heart, “Uncle Drew” is a personality-driven film. The plot is spare, and the storytelling gets super janky whenever screenwriter Jay Longino is asked to manufacture a new obstacle. But as wince-inducing as it can be to watch the film stall for time, or use the Rucker prize money to muster up some hollow conflict, the climactic streetball showdown pays off.

Howery steps into the spotlight, the NBA players do their thing, and the makeup team gets to dunk on the rest of the competition (even if they touched up the latex between every shot, it’s still insanely impressive how tight it stays even when Irving and co. are running fast breaks down the court). The message is clear: “You don’t stop playing because you got old, you got old because you stopped playing.” That’s easy enough to accept when everyone onscreen is having so much fun.

Grade: B-

“Uncle Drew” opens in theaters on June 29th.

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