A disaffected guy who's given to obnoxiously forlorn behavior like sitting on his roof alone (Daniel Radcliffe) meets a charming girl at a party (Zoe Kazan) who has an implausible job (cutesy animator) and can roll with his weird jokes. But she has a boyfriend! What's to be done?? There's really no reason for What If to succeed where dozens of similar indie rom-com efforts have failed, but it has some magic balance of a decent script and sparky chemistry between its stars to stand out from the pack.

Scripter Elan Mastai is working from the play Toothpaste and Cigars by TJ Dawe and Michael Rinaldi; the director is Michael Dowse, who showed promise with Take Me Home Tonight and Goon in 2011 but hardly needs to reinvent the wheel here. What If (titled The F Word in Canada, where it is set) is worst when it tries to do anything unusual, like having Chantry's (Kazan) big-eyed drawings flit across the scene in moments of quiet contemplation. It's not necessary. What If doesn't have to obnoxiously acknowledge the tropes of the will-they-won't-they romantic comedy. It just needs to gently nod at them.

Our hero Wallace (Radcliffe) is a dropout med student who lives with his sister and nephew and thankfully speaks in an English accent, which allows the actor to use his full range of charm. I would not have predicted Radcliffe as the most versatile star to emerge from the Harry Potter series, but he deserves applause for creating such a relatable, normal character in Wallace, who tries to set aside a crush on Chantry in favor of platonic friendship because she's in a serious relationship when they meet.

The pair's dilemma is an ancient one for the rom-com genre. Should they discard their obvious chemistry simply because of circumstance? What If doesn't take too many surprising turns to get where you know it's going to go, but it handles each turn with consummate professionalism. There's something especially relaxing about a film that obeys the strict act structure and character arcs of an ageless genre with such aplomb. What If doesn't need to obnoxiously razz its tired old story like the comparable but far inferior (500) Days of Summer. It just dresses it up very nicely.

Radcliffe and Kazan are what make thing work more than anything, but there's a realistic shagginess to the plot that helps. The Bruno Kirby and Carrie Fisher roles from When Harry Met Sally, ably filled by Adam Driver and Mackenzie Davis here, quickly point out to their best friends the protagonists that they should probably just end up together. But each character's dilemma is tangible enough that the whole thing doesn't just feel like an exercise in plotting. Wallace and Chantry's quick and deep friendship and unspoken attraction is a minefield navigated by many a 20-something in this day and age, and her accomplished boyfriend Ben (Rafe Spall) is easy to dislike but less simple to just dismiss.

There are scenes that feel too contrived, and Wallace and Chantry bat around dialogue that veers into embarrassingly cutesy territory. What If will undoubtedly grate for some, and a few groans rose from my audience when a character dashed onto a plane to make a grand romantic overture. But I was so on board from minute one, in a way I haven't been for a conventional rom-com since 2010's Going the Distance. Mastai's bouncy script deserves some credit, as well as Dowse's policy of largely getting out of his own way and letting the actors' energy carry us along.

But most credit should go to Radcliffe and Kazan, who are both doing their most natural work yet in their young careers. Kazan's skewering of the "manic pixie dream girl" in 2012's Ruby Sparks, which she scripted and starred in, left me cold, and Radcliffe until now has never really played just a regular guy onscreen. Both shed any pretention and excel as a result. What If might have a depressingly formulaic title and plot, but that just means I'm all the happier at its success.