The true-story raw materials for Rogue Agent would typically coalesce into either a breathless magazine article or a bloated streaming miniseries. Its degree of success as a feature film derives from Gemma Arterton- counterintuitive, perhaps, since Arterton historically tends to switch between a very British selection of background types. The rural manic pixie dream girl of Tamara Drewe, a smattering of WWII or Bond Girl pulpy types, and some obligatory and forgettable Hollywood love-interest roles: her mix of star quality and genericism is perfect for Rogue Agent, whose effectiveness depends on a lack of certainty over what genre, exactly, it will occupy.

The film opens in the early ’90s, with Robert (James Norton) recruiting several English university students for an MI5-driven mission to stop an IRA plot. From a tense, low-tech little intro—oddly plausible given the pressures of the time, priming a surprising jump to a decade later, when Robert meets Alice (Arterton), a successful lawyer. Her resistance to this pushy, chiselled car salesman veers between intrigued and wary, while his tenacity stays just barely more charming than creepy. Naturally, they begin seeing each other. But how will his secret-agent lifestyle fit in with this new relationship? Is this a movie about Gemma Arterton getting recruited for a stealthy mission, or Gemma Arterton navigating the perils of modern dating?

These questions may occupy some audiences longer than others. It helps, obviously, that Arterton’s particular Britishness somehow encompasses both stiff-upper-lip perseverance and singleton dissatisfaction. Her ability to appear equally likely to swoon or haul off and punch someone works well in a movie that gradually reveals itself as a different sort of procedural than it initially appears. Norton, meanwhile, has the showier role of the two leads, as a man blessed with the gift of providing reasonable-sounding excuses- also an actor’s curse when grasping for development. Any moment where Robert seems less than authoritative serves to undermine the character; sadly, the film skimps on such scenes, pushing a sense of creeping suspicion about Robert too hard, too soon.

This isn’t really Norton’s fault; Rogue Agent simply lacks the substance for a two-hour story. The relationship between Robert and Alice is easily the most compelling in the picture (lucky enough, as it largely constitutes the only one there). But directors/co-writers Adam Patterson and Declan Lawn haven’t hit on a way to dramatize the full story without breaking from that point of view. The movie regularly checks in on Sophie (Marisa Abela), one of the girls from the film’s grabby opening, and eventually follows up with Robert’s other recruits. By that point, though, the wider view feels perfunctory, with the bleary determination of put-upon Notting Hill copper Sonny (Shazad Latif) making sure his paperwork is in order.

At times, Rogue Agent feels reluctant to fully engage in the kind of deception that might make it a trickier, more “fun” piece of work; it’s almost too tasteful for its own good. Patterson and Lawn clearly have the chops to create something both visually detailed and slick; as with Arterton, the movie benefits from being able to look the part. Despite brief flashes of a grounded spy thriller or a psychological romance, the full product turns out so steady and even-keeled that it can’t really abide the full pleasures of genre — or of movie stars like Arterton, whose heartbreak and anger the movie regards with a curtain-twitching temerity.