Aid workers in the Balkans struggle to accomplish the simplest task in this ironically titled, pitch-black war comedy

There’s a little something of early Robert Altman in this enjoyably spiky comic war movie from the Spanish director Fernando León de Aranda: think of it as a second helping of M*A*S*H.

We’re somewhere in the Balkans in late 1995, and word of the ceasefire that has been thrashed out between the Serbian, Croatian, Bosnian and Yugoslavian foreign ministers in the United States has not yet filtered up to the rubble-strewn mountains where Mambrú and B, two international aid workers played by Benicio Del Toro and Tim Robbins, are doing their best to keep the local communities from self-combustion.

Overnight, the corpse of a very fat man has appeared in a very deep village well, which is bad news on two counts. Firstly, it has to be removed from the well within around 24 hours or the water will be rendered unpotable; and secondly, the corpse didn’t get there by itself, which means there’s more afoot here than cosmic bad luck.

León de Aranoa begins the film with one of the year’s great opening shots. We’re in the well looking upwards, while the corpse, suspended from a quickly fraying rope, spins gently in the blue-grey morning light. Hanging around, you quickly gather, is going to be a major theme here.

Mambrú (Del Toro) successfully winches the body up most of the way, but the rope snaps and down it goes again – just as the wild-eyed and woolly brained veteran B (Robbins) screeches on to the scene with Sophie (Mélanie Thierry), a recent aid-agency recruit with a delicate stomach, in the back of his jeep.

A Perfect Day (say the title with a sardonic shrug) follows this trio’s deceptively simple-sounding search for another length of rope with which to finish the job, although their numbers soon swell to six. Katya (Olga Kurylenko), a razor-tongued "conflict evaluator" and Mambrú’s ex-lover; Damir (Fedja Stukan), the group’s local interpreter; and nine-year-old Nikola (Eldar Reisdovic), a boy from the village, whom Mambrú rescues from bullies and who becomes a doe-eyed, sullen symbol of what’s at stake.

León de Aranoa adapted the screenplay from a novel by Paula Farias, and he has a knack for the kind of lightly ironised dialogue that’s the verbal equivalent of impatient drivers spinning their tyres. The group are in a hurry to set things right – particularly Mambrú, whom you gradually realise is viewing the outcome of this job, successful or otherwise, as a measure of his worth to the world at large.

But the locals and UN bureaucrats alike have time to kill and cross-purposes to talk at, which means this seemingly straightforward task becomes a Sisyphean slog of bartering, pleading, squabbling, and wheedling.

Del Toro and Robbins are well-served by the script, which offers them numerous chances to exercise their respective talents for dry and madcap comedy as the team trundle from village to village. It’s a pity that Thierry and Kurylenko aren’t afforded the same opportunities, but the female characters feel more like types than their male counterparts; an awkward boy-girl split that can be attributed only so far to the team’s boisterously macho esprit de corps.

But when the film gets going, it’s hard not to be bustled along with it, thanks mostly to León de Aranoa’s talent for punchy comic dialogue – doubly impressive, given this is his first English-language picture – and the plot’s habit of thwarting your expectations as to where the most morally upstanding course of action might lead. Mambrú’s best efforts might often come to nothing, but that doesn’t make them futile.