The exceptional French film Custody (★★★★, 12A, 93 mins) opens with a moment of peace and quiet: a county court judge sipping coffee in the dawn before a working day. Savour it, because this is the last respite anybody gets for 90 minutes. The coffee finished, the judge must leave her office and go downstairs – a descent that seems increasingly symbolic – to hear the preliminaries in an especially nasty custody battle. She’s soon up to her neck in the nitty-gritty, landing an eyeful of the state of play in the apparently ongoing battle of the sexes. The overbearing father; the seemingly fragile mother. Not for the first time recently, we’re left picking sides. At first glance, the matter would seem open-and-shut. Papa Antoine (Denis Ménochet, the looming patriarch of Mary Magdalene) has both the dimensions and the demeanour of an aggrieved prop forward, an impression his opponent’s lawyer backs up with claims of harassment and physical violence against the couple’s two children. Yet maman Miriam (Léa Drucker) seems cold and distant – or just chilly around her ex? – and snatches, without thanks, at a care package thoughtfully prepared by her former mother-in-law. In a weird way, the two have arrived at an equality: both are approaching the end of their tether. The only question left is which, if either, is best placed to do right by their offspring. The fallout brims with subtly heartbreaking detail: those evasions and omissions designed to protect one life from another, the nervy welcoming parties gathering around front doors to ensure nobody’s laid fingers on the children, who are shuttled between guardians, or that no one’s storming up the garden path behind them. Given writer-director Xavier Legrand’s emotive close-ups, we might well side with the kids, who surely deserve better than this. Yet Legrand locates an exhausted sadness in Ménochet and Drucker that moves us in different ways: a desire not to have to go to these extremes to hold on to something, or to return to the normal civilities, impossible as those may now be. Legrand’s achievement lies in taking the overarching fraughtness of these situations and shaping it into a thriller framework. Here the film is made gripping by the fact that every character is unhappy, there are no easy solutions, and the complications keep redoubling: one superbly suggestive sequence spies the couple’s daughter unboxing a pregnancy test, longing to start a family to replace the one blown to smithereens. Custody is a series of such bombshells, waiting to explode; its heart-in-mouth finale involves no less than an outbreak of domestic warfare. Savour that coffee, then, because it’s quickly drained. Legrand drags us as close to real life as fiction will allow, messy and painful though the experience may be.