Relay may be one of the most gripping movies of the year despite its dry premise thanks to mesmerising central performances and slick direction.

Unfortunately, a substantial portion of the audience are likely to leave the cinema furious thanks to a sharp left-turn in the final act.

There won’t be any spoilers here, but rest assured this stripped-down whistleblower thriller heads in a direction you won’t expect, so make sure to go in with measured expectations.

The Night Of’s Riz Ahmed takes on another unique role as Ash, a fixer who uses a relay phone service for the deaf to anonymously aid clients who have unearthed evidence of corporate corruption and cover-ups.

When a new client, Lily James’ in-over-her-head Sarah Grant, contacts him to do the exact opposite – deliver incriminating documents back to the company she stole them from and lay low – he wrestles with his responsibilities to keep her safe and do the right thing.

Filmmaker David Mackenzie once again proves himself one of the most versatile mid-budget directors after his gritty Western crime epic Hell or High Water and historical drama Outlaw King.

His no-nonsense approach works wonders for Relay, a tight espionage thriller that sounds complex on paper but quickly gets under your skin thanks to the magic of the two leads.

Films about exposing both government and corporate corruption have always been quietly enthralling, even if they don’t feature any breakneck action or suspense scenes common to more traditional thrillers.

Think Michael Clayton, Erin Brockovich, or Todd Haynes’ recent sinister masterpiece Dark Waters – admittedly weightier films about threats to public health than Relay, which takes the concept and adds some Hitchcockian twists to various degrees of success.

Thankfully, it mostly works, as most of the near two-hour runtime had me on the edge of my seat as Ash and Sarah hurtle around New York trying to deliver documents, get them back, realise they’ve made mistakes, put them somewhere else again, rinse and repeat until the audience’s knuckles are chalk white with tension.

Meanwhile, a group of goons-for-hire led by smug Dawson, played by Sam Worthington at his most punchable, and Willa Fitzgerald’s sparky Rosetti, are in dogged pursuit with every turn they make.

Some of Relay’s best moments arise when the relay service is used to outsmart and belittle Sarah’s tails, including a superb early sequence in an airport that expertly demonstrates Ash’s resourcefulness.

What makes the film feel most like a modern Hitchcock is the brilliance of conveying chemistry between Ash and Sarah, two characters who almost never meet in person. Not only does their burgeoning connection develop over phone calls, but most of the time Ash is speaking through someone else’s voice, a true testament especially to James’ talent to convey a genuine spark through sparse dialogue and subtle reactions.

Ahmed, meanwhile, is just as captivating as an almost silent protagonist, darting his way through nerve-wracking set-ups and narrow escapes with a stoic smoulder that should make James Bond’s casting directors take notice. He also brings endless pathos to the role when Ash’s personal hang-ups come to the forefront, delicately portraying shame at his own alcoholism after being raised Muslim.

For most of Relay, this highwire act of tentative romance and tension is served brilliantly by an economically bare-bones yet frequently witty and nerve-wracking script, making you feel every sympathy for Sarah’s desperate situation while Ash silently contemplates whether his next move is in his client or the public’s best interests.

However, this bubbling pressure bursts almost immediately with a third-act twist that crumbles under scrutiny and abandons much of what made the first hour and a half so magnetic. Perhaps the aim was to wake up audiences who may have lost patience with the low-key cat-and-mouse game, as this is also where the stakes are needlessly raised with a claustrophobic fist-fight in a speeding van (admittedly well done) and some obligatory shoot-outs (less so), but there was no need.

Maybe some audiences will find Relay kicks into high gear after this unexpected reveal, and there’s a chance it will hold up better on a rewatch, especially as the first two-thirds certainly have the energy to sustain plenty of repeat viewings. Sadly, though, the attempt at rug-pulling for me only succeeded in ending this otherwise sensational thriller with a feeling of cynical deflation.

Relay is in UK cinemas from Friday, 31st October.