Warning: This review discusses the film’s ending.

The latest from The Last King of Scotland’s Kevin McDonald, backed by public money and funds from BFI subscribers who’d want a little more from their indigenous cinema, is an oddball adaptation of Meg Rosoff’s young adult novel, in which a insolent and troubled American teen is sent to live with her English country cousins, only to find herself in the centre of a Raymond Briggs-type post-nuclear bid for survival in what remains of England’s green and pleasant.

It isn’t clear why the father of Saoirse Ronan’s Daisy would send her to Britain, or indeed anywhere near Europe, in the middle of a high profile security threat (there’s footage of Paris aflame and troops on the ground when her plane touches down), it’s never explained, and it appears as if America’s a safe haven from the unidentified terrorists as a consulate lackey arrives not long after the bomb drops with a ticket for her safe return, so perhaps she should have gone to stay with relatives closer to home, but nevermind; she comes and for a while it’s an agreeable little setup.

In early scenes, in which a damaged Ronan makes hard work of ingratiating herself with her happy and playful anglo-fam, McDonald develops some chemistry between the family; enough that we might care for these kids once civilisation goes boom. The only relationship that lacks any substance, any spark, is the one between the sexually aware Ronan and her muscular cousin; a near mute George MacKay, and that’s a pity, because it transpires this is the union that’s going to drive the plot and provide the story with its emotional core.

As it isn’t clear why Daisy and cousin Edmond have fallen in love, or indeed why they don’t have reservations when they’re blood relatives, it’s hard for us to understand why the love sick American would opt to stay in a post-apocalyptic country, rather than leave a new relationship predicated on a look, a finger suck and perhaps ten words. Things at home must be pretty bad, you think. Consequently when the couple are separated, a couple we care nothing about, it’s hardly a wrench. Our hope, which like those of the characters fades as the movie progresses, is that McDonald is minded to put Daisy through the wringer in her bid to be reunited with her inexplicable crush; that the horrors of war will be stark and apparent and make us want to hold our loved ones a little tighter; but this, despite one use of the C word and close ups of bullet-holed adolescents, remains a young adult war in conception. It’s so low key as to be almost uneventful; a World War 3 suitable for tea time audiences with sweary parents: it is, in short, stultifyingly unambitious.

Still, you say, at least this is a movie with a strong and resilient female lead, but before you uncork the champagne, take stock of the dénouement: the couple reunited, poor Edmond scared by the wartime experiences that we’d dearly loved to have seen but didn’t, and a grateful Daisy ready to revive civilisation along some reassuringly familiar lines. “Now I use my will power to care for you and love you” she tells her beau, while he meanders around, looking useless. Good old nuclear war, you think; the best things survive – namely cockroaches and patriarchy. It’s an ending that girls everywhere will take to their hearts.