The third act, home to the show’s resolution and finale, saves “Theater Camp,” a show that’s lacked anything resembling a “show stopper” up to its last scenes. The script is cleverly conceived to conceal the plot’s “big original musical” from the viewer, and the true depth of the talent of this Camp Adirondacks summer camp for kids ages 8-13 (or so) before that Big Finish.

There are laughs in the first two acts, giggles at the spotlight-hogging tyke “types,” the self-serious teachers, the theatricality of one and all. But that finale loses the “camp” and remembers the “show” in a “Let’s put on a show” musical. That’s what allows a marginal comedy to let you leave the theater with a smile and an affirmation that yes, these kids have found their tribe, have found their passion and have figured out that they have “it.”

Based on a cheerfully cheesy and spot-on spoofy viral short film of a few years back, this Molly Gordon, Nick Lieberman and Noah Galvin project is about a struggling camp named “AdirondACTS” located in the you-know-where mountains. Somehow, founder Joan (Amy Sedaris) and manager Rita (Caroline Aaron) & Co. have kept it funded and afloat, welcoming kids “who don’t fit in anywhere else” — thespians.

But a recruiting/fund-raising trip leads to Joan having a seizure in the middle of a middle school’s musical. Those darned strobing lights resulted in “The first ‘Bye Bye Birdie’ related injury in Passaic County history,” Joan’s coma, an inter-title drolly informs us.

Yes, this is a mockumentary. And yes, the filmmakers inform us in another title, their “star” is incapacitated before the summer’s even started. But the show, and the mockumentary, must go on.

Not to worry. Joan’s adultish son Troy (Jimmy Tatro, hilarious) will take over and get them through the season. Troy a delusional “financial guru” vlogger and sometime DJ with strong bro-dunce energy. He can’t read the room when he introduces himself to the campers, many of whom have been there before. He can’t win over the staff, many of whom he fired to cut costs. He can’t make the mortgage or power bill payments because he’s uh, not opening the mail.

Ben Platt and co-writer/director Molly Gordon play the heart and soul of AdirondACTS, life-bonded friends who met at camp there many years before and who tied their fates together — professionally (she figured out he’s gay) — and return here every year to teach.

They make a big production out of everything, delivering the season’s line-up of shows — “Damn Yankees” and “Cats” included — in a sort of “Forbidden Broadway” musical vamp. Their speciality? Every summer they whip up an original musical for the best kids in the camp to perform, and previous shows like “A Hannukah Divorce” will be joined by their work-in-progress inspired by their boss and inspiration’s coma, “Still, Joan.”

“Camp” co-writer Noah Galvin plays the lights/sets/electrics guy who keeps everything running — barely — and who harbors a hidden pain. Glenn had Broadway dreams and wanted to be in the spotlight himself at one time.

A rival Camp Lakeside owned by a hedge fund operation would love to buy AdirondACTS, and fetching Barnwell Capital shark and camp director Caroline (Patti Harrison) could be just the one to tempt Troy into some blunder that kills off his mother’s dream.

The kids are a sparkling assortment of little narcisissists, but they’re in the background mostly. This little egomaniac is “an example of a kid who started too early” and that one isn’t interested in the greasepaint and spotlight. He’s eight and hustling himself into a career as an agent.

The adults are center stage here, the preening dance teacher (Nathan Lee Graham) and bitchy costumer (Owen Thiele), and the movement/masks/stage combat newcomer Janet (Ayo Edebiri) who is such a fraud only Troy could have been fooled into hiring her.

The mockumentary format is barely present and half-hearted, at best. And much of the humor will play as terribly inside baseball or off-off-Broadway or community theater to people who have never tiptoed into that world.

The teacher who resents his younger and more talented student, the theatricality of one and all, the self-importance and vanity behind a motto that holds that “summers come and go, but what happens on this stage? That’s eternal!” — it’s worth a giggle, an eye-roll and the occasional spit-take in recognition from anyone who’s ever done “theatre.”

Everybody’s pretty good. But Youtube-famous actor Tatro is amusingly clueless, “Booksmart” veteran Gordon and “Pitch Perfect” and “Dear Evan Hansen” alum Platt pair up beautifully and really get across the low-stakes/life-and-death importance of the work of those who truly LIVE the theater.

Still, it isn’t until the “camp” is shoved upstage and the kids and their Big Theater Show step downstage and into the light that “Theater Camp” finds its heart and hits its best notes.

Cast: Ben Platt, Molly Gordon, Noah Galvin, Jimmy Tatro, Ayo Edebiri, Nathan Lee Graham, Owen Thiele, Patti Harrison, Caroline Aaron, David Rasche and Amy Sedaris.

Rating: PG-13, some profanity, drug jokes

Credits: Directed by Molly Gordon and Nick Lieberman, scripted by Noah Galvin, Molly Gordon and Nick Lieberman, based on the short film by Nick Lieberman. A Searchlight release.

Running time: 1:33