“My Old Ass” is keeping a couple of secrets, one of them a doozy.

Know that it’s pretty easy to guess that big secret as writer-director Megan Park’s inventive and comedic coming-of-age tale rolls along, but that doesn’t make the film any less satisfying.

That is no small part due to the performance of its lead, Maisy Stella, and key supporting player Aubrey Plaza, both of whom shine in the film, which is going wide Sept. 27 after a limited theatrical release earlier this month.

Stella (“Nashville”) is Elliott Labrant who is turning 18 with a night with besties Ruthie (Maddie Ziegler) and Ro (Kerrice Brooks), in which they will camp out and get high on psychedelic mushrooms. (Her cranberry-farming family waits for her at home with a cake, but she never picks up when they call.)

After chugging her tea, the shrooms eventually kick in, and while her friends have vastly different experiences, she is visited by the 39-year-old version of herself (Plaza). The latter character — eventually slapped with the nickname “My Old Ass” — must convince Elliott by showing her a shared scar. (But the gap in her teeth? Hey, maybe wear your retainer, Elliott.)

Old doesn’t want to reveal much about the future — she won’t tell Elliott what company will become the next Apple for investment purposes but does hint that salmon is a thing of the past — aside from she’s going for her Ph.D. (This does not excite Elliott.) The 39-year-old version — who, Elliott, is not middle-aged, thank you — mainly implores her to stay away from anyone named Chad.

Of course, Old is cryptic about this, not giving Elliott any details about whatever this Chad character may do. Still, no problem, Elliott figures; she’s planning to have a blast with her friends in the few weeks she has before going off to college. Plus, if it’s some romantic situation, that’s no big deal — Elliott for years has identified as gay and has just started hooking up with the girl she’s been crushing on all summer.

Elliott takes Old’s advice to spend more time with her family — to be nicer to her mom (Maria Dizzia) and to try to bond with the older of her two younger brothers, Max (Seth Isaac Johnson, “The Killing”), who long has planned to take over the farm and in whom she’s never been very interested.

But this Chad (Percy Hynes White) does appear, literally emerging from the water where she’s swimming in the nude. (It’s very murky down there, so he couldn’t see anything, he promises.)

Elliott does her best to ignore the long-haired Chad, which is difficult because he is helping out at the farm and treated well by her family. Plus, he’s frustratingly nice to her and, like, has this annoyingly symmetrical face.

Fortunately, for her, Old put her number in Elliott’s phone, and — even to the senior’s version’s surprise, they are able to talk and text. However, when Elliott really needs her, she stops responding.

“ My Old Ass” is one of those movies you just go with, choosing not to think much about its supernatural element. That’s particularly easy because it’s a blast to hang out with Stella, whose Elliott is relatably imperfect — she is, after all, a teen — but possesses a vibrant personality you can’t help but enjoy.

While Hynes White (“Wednesday”) checks the necessary boxes as Elliott’s confusing first boy crush, it is the always interesting, often quirky-cool Plaza (“Emily the Criminal,” “The White Lotus) is more impactful (even as she does, in fact, look little like Stella). She is mesmerizing in a late scene that, while highly emotional, is nicely understated.

It’s one of several nicely executed by Park, who portrayed Grace Bowman for seven seasons in “The Secret Life of the American Teenager.” “My Old Ass” is her second feature as a filmmaker, following 2021’s excellent “The Fallout,” which she wrote and directed.

Park isn’t the first person to make a film that explores the idea of whether you should change the future — that is if you even can — but she’s found a fresh way to do it.

And given the strengths of her first two films, we’re greatly looking forward to whatever she makes next.

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‘MY OLD ASS’

3 stars (out of 4)

MPA rating: R (for language throughout, drug use and sexual material)

Running time: 1:28

How to watch: Now in limited theatrical release, expands nationwide Sept. 27

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