Directed by Ken Loach. Starring Mark Womack, Andrea Lowe, John Bishop, Jack Fortune, Geoff Bell, Talib Rasool, Craig Lundberg 16 cert, IFI/Light House, Dublin, 108 min

YOU WOULDN'T be out of line if you described the latest Ken Loach film as a thriller. Following an ex-squaddie as he investigates the death of his pal in Iraq, Route Irishfeatures more narrative tangles than a post-Watergate conspiracy drama. Cars are bugged; men wearing suits lurk in doorways. Yet there's no mistaking the film's parentage. Loach's invigorating Marxist anger burns through every frame.

Featuring many flashbacks, Route Irishconcerns the killing of Frankie (comic John Bishop, who makes an excellent straight man), a British "private security consultant" – that's a mercenary to you, me and Noam Chomsky – on the road out of Baghdad Airport. Back home in Liverpool, former SAS man Fergus (Mark Womack) begins probing into the relevant security firm's worrying disregard for everyday morality. He feels particularly guilty, because he persuaded his friend to take the lucrative job.

A video is discovered on Frankie’s phone showing an entire Iraqi family being murdered in mysterious circumstances. The lads’ increasingly sinister employers (they might as well be called Evil Capitalist Stooge Inc) offer an explanation, but Mark is well on his way to uncovering a despicable plot.

Though the publicity material suggests otherwise, Route Irishtakes place almost entirely in Liverpool. Using skills learnt in the SAS, Fergus proves more than a match for his complacent enemies.

Working with writer Paul Laverty, now his most regular collaborator, Loach lucidly demonstrates how private interests have taken over the occupation of Iraq. If war is a business, then the harshest dictates of capitalism are the only rules worth applying.

If only the mechanics of the picture were as effective as its clear-headed political arguments. Too many of Fergus's techniques seem roughly, incongruously dragged from a bad James Bond novel. A faintly ridiculous water-boarding sequence provokes more groans than wails of outrage. Much of the dialogue would seem clunky in an instalment of The Fast and the Furious.

For all that, Route Irishremains a modestly gripping slice of political melodrama. And now more than ever, audiences need a good metaphorical clip round the ear with a rolled up copy of the Morning Star.

Readers searching for belated St Patrick's Day fun should be aware that, the name of that thoroughfare aside, Route Irishhas nothing to do with the old country. Unless we're allowed to claim Liverpool. Are we?