Jenny Slate and Charlie Day deserve better than “I Want You Back,” a leaden rom-com that gives them a shot at being funny, charming, and sweet, only to squander it scene by scene. They’re an inspired pairing for casting, playing two unlikely friends bonded in their heartbreak. Slate’s Emma has just broken up with a fitness trainer named Noah (Scott Eastwood) after 18 months; Day’s Peter just got the boot from English teacher Anne (Gina Rodriguez) after six years of being together, along with condescending words about how he’s stuck in life. A tearful Emma and Peter meet in a stairwell, their unfulfilling jobs just floors away from each other, and decide to be supportive friends in mourning. 

Realizing their shared jealousy and not their inability to move forward, they crack a scheme—they’ll ruin the new relationships of their partners so they can get back with them. She will get Anne’s new boyfriend and drama teacher Logan (Manny Jacinto) to cheat with Emma by volunteering for his school play, and Peter will get inside Noah’s head to break up with his new girlfriend Ginny (Clark Backo) by becoming Noah’s best friend. They won’t have to look at their respective exes brandish their happy relationship on Instagram, and they won’t have to worry about the daunting notion of finding a new person. They won’t have to face the reason they were broken up with. 

It’s an absurd scheme, so vivid in its delusion and desperation, and it’s a great set-up for the kind of rom-com meant to be paired with a pint of ice cream. But director Jason Orley’s film doesn’t take advantage of the ample room that comes with the genre; it’s almost like the movie is afraid of getting darker with its break-up screwball concept, even though the pain and denial it’s riffing on is so true. The friendship that develops between Peter and perfectly amiable Noah isn’t especially charming, and the courtship that Emma tries to fling onto Logan, under Anne’s watchful eye, feels more forced than its design. “I Want You Back” takes a cheaper way out of its conspiracy, by not letting these unsuspecting targets have much of their own personalities, minimizing the performances of its intriguing ensemble. All so that we don’t really question whether Emma and Peter are being bad people, or face head-on the manipulation of their plot. 

“I Want You Back” is almost two hours long, but it doesn’t have nearly enough comedic momentum to justify that runtime. So many scenes from this script by Elizabeth Berger and Isaac Aptaker play out like scenarios needing a punch-up, whether it’s jokes about Peter being out of shape, or Emma’s strange placement volunteering for the school play. And it’s a big eye roll when the movie jokes about how Logan’s artistic dreams have made him a pretentious director for 12-year-olds, one of many easy targets.