With its intriguing premise, the New York–set Relay is a throwback to old-school political and spy thrillers, where the process of the work was often as, or more, engaging than the characters themselves. In this sense, it succeeds. Yet it also has a foot in the gimmick-laden world of last-act surprise films, and the reveal at its ending stumbles badly.

Sometimes whistleblowers have second thoughts and want to return the material they have purloined as evidence. In moments like this, they turn to someone like Ash (Riz Ahmed), a go-between for the leaker and the company. Silent and inscrutable, Ash communicates in a unique way: through relay operators, an actual system used by those with verbal or hearing disabilities. A caller types on a keyboard, and the operator relays the words aloud to the hearing person on the other end. It is completely confidential, with nothing recorded. This suits Ash quite well.

It also suits Riz Ahmed. With his wiry physique and puppy-dog eyes, he excels at playing characters who appear externally calm but are inwardly jumpy as hell. Roughly 80 percent of his screen time—and he’s in most scenes—is nonverbal. Ash is simply doing his job, and director David Mackenzie makes that job look fascinating. Essentially employing the tools of spycraft to keep his clients safe, Ash becomes something of a protector. From a few monologues, you glean a sketched-out idea of why he does this work, but Ahmed, one of his generation’s best actors, fills in the rest.

Lily James, as Sarah, Ash’s client, is less well served. She does fine with what she’s given, but it isn’t much. A scientist, Sarah stole information from a biotech company after her warning about a product’s unintended side effects went unheeded. The company hires thugs to follow her and blow up her car as she reports the information to the authorities. An attorney refers her to Ash, and the two communicate minimally through relay operators. Still, a relationship develops between these two lonely people. Yet Sarah has even less backstory than Ash. She lives in a lovely apartment, and Ash glimpses photos of her friends on Facebook, but she otherwise seems a hermit. For most of the runtime, she serves largely as an object of Ash’s growing affection.

Those hired by the company to harass her and retrieve the stolen documents include Sam Worthington as the stoic leader Dawson and Willa Fitzgerald as Rosetti, his savvy second-in-command. The fun of Relay lies in watching the cat-and-mouse game as Ash and the team strive to outwit one another. Here the film’s debt to spy and procedural thrillers shines. Both Ash and Dawson are well versed in deception and surveillance. Watching Ash pit his lo-fi technology and street-level resourcefulness against Dawson’s high-tech operation is a delight.

For the first two-thirds, you’re watching a solid thriller—well shot, well acted, and smartly paced. It isn’t groundbreaking, but it is enjoyable and recalls the way thrillers used to be made. Watching the dance between Ahmed and James, two charismatic performers, is a pleasure, even if the beats are predictable. Then comes a third act that detonates the whole thing.

A good final reveal makes the audience reassess what they’ve seen in a new light, as in The Sixth Sense. But for that to work, the script has to be airtight. Relay’s is not. Its twist plays like a “wouldn’t it be cool if” idea that no one bothered to rework the script around. The ending doesn’t match how the characters previously behave, and it opens major plot points to questioning. Up to then, the film is smart, specific, and detail-oriented in showing how its characters act, live, and do their jobs. Suddenly, all that goes out the window, and disappointment sours what is otherwise a strong film. Still, until the letdown, there is plenty to enjoy. If you brace yourself for the finale, you may still come away appreciating Relay.