In Ken Loach’s conspiracy thriller, an American human rights activist (Frances McDormand) and a no-nonsense UK police officer (Brian Cox) work tirelessly to uncover the truth behind a suspicious death — but this is no rousing triumph of good over evil, only a bitter pill about corruption and complicity. Human rights lawyer Paul (Brad Dourif) and girlfriend Ingrid (McDormand) are in Troubles-era Belfast wrapping up their report into the UK government’s shoot-to-kill policy when Paul is shot dead by mysterious assailants while on his way to speak to an informant. Police investigator Kerrigan (Cox) is flown in from England to look into the death; Cox plays him as a stickler for the rules and fierce agent of justice, inspired by a real-life official who was abruptly suspended from the police before he could deliver his findings into the UK government’s alleged death squad in Northern Ireland.

But the network of high-level corruption that Kerrigan and Ingrid uncover poses a direct challenge to those values, ultimately forcing him to choose between revealing the conspiracy — and risk destroying his life — or concealing the truth and betraying his ethics. A fascinating look at the slippery slope to complicity, this is a gripping, unabashedly ideological conspiracy thriller blending Hollywood polish with gritty reality.

The politics of firebrand filmmaker Loach made this a divisive winner of Cannes’ Grand Jury prize and stirred up much debate on release in the UK, but what’s unarguable is the power of Cox’s performance. There are notes of Logan Roy in the fierceness with which Kerrigan hunts down the truth, a high-mindedness that only makes the film’s ending all the more gutting. Not an uplifting performance, then, but undoubtedly the compelling heart of this sobering film.