Save the Date may be a charming romantic comedy, but it’s a kind of horror movie, too. In the film’s pivotal early scene, Sarah (Lizzy Caplan), a bookish young artist who has just moved in with her nice-guy rocker boyfriend Kevin (Geoffrey Arend), finds herself at a loss for words when he suddenly proposes to her onstage during a concert. Already feeling a little uncertain about the whole moving-in-together thing, Sarah rejects Kevin — right there, in front of all his fans. Ashamed, she leaves the concert, as the film cuts to him in a dark room backstage, breathing heavily, terrified, and virtually unable to speak. Director Michael Mohan intercuts between the two of them, but his cutaways to Kevin seem to hold for an unusually, even cruelly long time. They don’t, of course, but it feels that way: We take in the full scope of this young man’s anguish, and, for a moment at least, we’re not in easygoing romantic-comedy-land anymore.

Save the Date is a gentle movie in its broad outline — full of pleasant music cues, likable characters, and a narrative arc that bends towards optimism and forgiveness — but in its particulars, it betrays an emotional mercilessness that sets it apart. Post breakup, Sarah tells her soon-to-be-wed sister Beth (Alison Brie) that she’s happy just being who she is and that she doesn’t want a relationship constraining her; even so, she starts to date mopey, likable, and only somewhat stalkerish marine biologist Jonathan (Mark Webber), a guy who was actually there at the concert when Sarah rejected Kevin’s proposal. Mohan intercuts the lovelorn Kevin’s melancholy concerts and pathetic hookups with Sarah and Jonathan finding themselves ever more drawn to each other, as they have great sex and talk about how perfect they are for each other. So, we get to watch a lovelorn guy fall to pieces while, miles away, the girl who ripped his heart out appears to find true love.

That may not sound like something anybody who’s ever been through an actual breakup would want to watch, but amazingly, this unforgiving quality doesn’t appear to be a miscalculation. (The film was co-written by the cult cartoonist Jeffrey Brown, with whose work I’m not familiar, but who is reportedly good at depicting awkward everyday situations. I believe it.) We like all of these characters, and it is a measure of the film’s accomplishment (and honesty) that we’re happy for Sarah. That’s not just because she’s our protagonist, but also because Caplan, an actress who has an uncanny knack for simultaneously conveying both toughness and vulnerability, perfectly portrays the character’s muted cluelessness. Sarah doesn’t quite know what she wants; she just every once in a while realizes what she doesn’t want. She’s a mess without ever quite acting like one, which makes her a pretty good stand-in for most of us in the audience. It’s an amazing performance, without ever seeming like one.

There’s actually a whole other plotline, too. While Sarah tries to navigate the romantic chaos of her life, her sister Beth tries to keep herself focused on her impending nuptials to Andrew (Martin Starr), one half of the band Kevin plays in, which of course complicates things further. There is a broader point being made here: The two sisters are each on the cusp of something new in their romantic lives, and the film betrays a kind of unspoken horror at the idea of commitment. Well, not entirely unspoken: At one point Andrew notes that once he’s married the only time he will ever not be with Beth is when one or the other of them is dead.

The commitment angle is an interesting one, but it remains largely cerebral. Sarah’s breakup story is the more compelling thread here. (I saw the film almost a year ago, at Sundance, and watching it again, I was surprised to realize I’d completely forgotten about the whole Beth storyline.) It’s certainly the angle that will hit the closest to home for most viewers. Save the Date works best when it’s getting under your skin, and it does that when it’s capturing the queasy halfway point — part sadistic, part bittersweet — of still loving somebody while trying to move on to someone new. It’s a kind of subtlety that movies, especially American movies, rarely do well, but this quietly unassuming, secretly brilliant little charmer nails it.

Movie Review: Save the Date Is a Horror Rom-Com