“I Want You Back” is a rom-com that sort of drifts along, not quite petering out, not exactly sparking to life, until that magical moment when Pete Davidson shows up.

No, nobody said that. Ever. But with the Lorne as my witness, it’s true.

Davidson shows up at a girls-pick-up-guys party, a punk given to dating well beneath his age. And with a little “molly,” a hint of edge and a dose of gonzo, he helps the film earn its R-rating and find something like its mojo. There are laughs, and a little sexual slapstick enters the picture.

It doesn’t save this “let’s break up our exes’ new relationships” rom-com, a variation on an “Addicted to Love” model. It’s not down and dirty, and not remotely as sophisticated as its “When Harry Met Sally” soundtrack suggested they wanted it to be. The pace is New Orleans brass band procession funereal, and it goes on well past its payoff. But it does give us a taste of what could have been, before the bitter aftertaste sets in.

Jenny Slate is Emma, and we meet her just as personal trainer beau Noah (Scott Eastwood) is dumping her in the middle of her second Lover’s Punch. On the other side of Atlanta, Anne (Gina Rodriguez) is giving bubbly Peter (Charlie Day) the heave-ho at her nephew’s birthday party.

Emma and Peter “meet cute” in the stairwell of the building where both work, where he catches her weeping, and she notices he’s doing the same.

Over drinks, cell phone video reminiscences of their “couple” days and drunken karaoke, they become each other’s “sad sister,” the one they call when they’re tempted to call their ex. And at some point during their dinners, nights at the movies and such, a “Strangers on a Train” plot is hatched. She’ll “seduce” the middle school drama teacher Logan (Manny Jacinto, pretentiously funny) whom Anne took up with. And he’ll befriend Noah to trash talk his new lady love, the fetching pie maker Jenny (Clark Backo). What could go wrong?

“This is like ‘Cruel Intentions,’ only sexier!”

The former stand-up comic Slate banters well, and still can do the cute and “nobody loves me” thing with panache, if not a lot of laughs. Day’s lost some of his “Always Sunny in Philadelphia/Horrible Bosses” fastball and aged out of that manic screeching thing he does when he’s put out.

Together, they set off so few sparks that you remember why legacy studios gave up on each of them (Slate fares better in indie films) years ago. They’re cute support, not leading lady/man material.

Eastwood and Rodriguez aren’t known for comedy…with good reason.

Which leaves the picture’s light touch in the hands of screenwriters who don’t have enough jokes and a director who lets things go on and on when somebody should have said “FASTER” to the cast and “FUNNIER” to the writers.

So Davidson comes in and kills. And that’s after Jamie Gertz, playing Peter’s penny pinching nursing home conglomerate boss, has landed the only decent laugh in the first act.

“These people are at death’s door and we are spending WAY too much money to feed them!”

If they’d workshopped this to figure out more to do with Day, or simply made the film from Slate’s point of view, things might have improved. Slate has some sweet scenes befriending a depressed middle school kid.

And when Slate’s Emma is corralled into subbing for Audrey in the middle school’s “Little Shop of Horrors” because she lied to director Logan about having “played” her in high school, all part of her planned “seduction,” “I Want You Back” shows everybody involved and everybody watching it could have been funnier, sweeter and darker at the same time.

Rating: R for language, sexual material, some drug use and partial nudity

Cast: Jenny Slate, Charlie Day, Gina Rodriguez, Scott Eastwood, Clark Backo, Manny Jacinto and Pete Davidson.

Credits: Directed by Jason Orley, scripted by Isaac Aptaker and Elizabeth Berger. An Amazon release.

Running time: 1:52