There’s only one really good reason to make a body-swap movie. Yes, OK, thematically they’re meant to teach us to be more empathetic and think about living in someone else’s shoes, blah, blah, blah. But a body-swap movie has one true purpose, and that’s to make us laugh at someone acting like someone else.

“Freaky Friday”: a mom and her daughter switch places, and it’s funny. “13 Going on 30”: a teen girl ends up in the body of a mean, gorgeous magazine editor, and it’s funny. “Big”: a boy is in a man’s body, and it makes us laugh. “Jumanji”: four teens end up in the bodies of a beefcake, his sidekick, a dork and a hot lady, and it’s hilarious.

“It’s What’s Inside” is trying to be hysterical, too, but with less amusing results. The premise of Greg Jardin’s comedy is relatively promising: Seven 20-something friends from college converge on a beautiful estate that belongs to one of them, who is getting married the next day. They’re going to have one last night of partying before the wedding. Two are a couple who’ve been fighting; one of the guys is carrying a torch for one of the girls; another of the girls, the blonde, has become an influencer; the others also have their own lives now.

Then an eighth friend shows up, a guy nobody’s heard from in a long time — not since a terrible and unfortunate event years earlier at a college party. He’s bearing a weird box containing a strange device that, it turns out, can cause them to trade bodies. He proposes a Mafia-style party game, using the box, and things go off the rails really fast.

“It’s What’s Inside” feels a little reminiscent of the much better “Bodies Bodies Bodies” (2022), a horror-comedy about a bunch of intoxicated, very online youths in a rambling house playing a crazy party game. But where that one zigged and zagged, “It’s What’s Inside” plods straightforwardly. Even the twists feel obvious and not all that interesting, more the fulfillment of plot points seeded early on rather than startling turns of fortune.

The lumpiness is baffling, to be honest. This concept has promise. Some of the fault is in the casting; while half of the actors give performances that are fun and quirky, the others feel as if they’re reading lines, and not particularly well. A little of it is also in a self-consciously showy filmmaking style (weird lighting, fast cutting, freeze-framing) that doesn’t add anything to the film. At times, it distracts, or maybe subtracts.

But the roughest thing about “It’s What’s Inside” is that it simply misses what makes body-swapping movies fun. There’s little humor to be found in juxtaposition here.

It might be funny to see your own friend, someone you’ve known for years, appear behind the face of another friend. But to an outsider, those differences would be fairly opaque, as they are in the movie for the audience. These are eight characters who are not very different from one another. Sure, one is an influencer, one seems like a stoner, one is a party guy, one is uptight — but with so many personalities in play (personalities that remain undeveloped), it’s not just confusing but boring.

So we can’t know them well enough to know how they’re supposed to act, and thus to notice when they act a different way. We know we’re supposed to laugh, but we haven’t been introduced to them long enough to know why. It’s a shame, because a body-swap comedy usually promises a good time. But this party isn’t much of a party. In the end, you might rather wish you hadn’t been invited.