American teenager Daisy (Saoirse Ronan) arrives to stay with relatives in Britain at perhaps the worst possible time in “How I Live Now.” (Magnolia Pictures)

Saoirse Ronan, whose ethereal, translucent beauty made such an impact in films from “Atonement” and “The Lovely Bones” to “Hanna,” bleaches and hardens her look to play Daisy, the tough lost girl at the center of “How I Live Now.” Adapted from the young adult novel by Meg Rosoff, Kevin Macdonald’s uneven near-futuristic love story may slake YA thirst before the next “Hunger Games” installment arrives, but its interest lies chiefly in the stretch it represents for its teenaged star.

As the film opens, Daisy, a quintessentially American teen, glumly arrives in England for a summer stay with cousins, at the directive of her distant father. As with “Atonement,” Britain seems to be on the brink of war; in this case, however, it’s World War III, with bands of never-named terrorists bombing Paris and, eventually, London itself.

Stranded at the cluttered country house of her relatives, Daisy joins forces with her cousins to find sustenance as England becomes a military state; eventually they’re separated, and her goal becomes to survive and reunite with Eddie (George MacKay), the stalwart, handsome cousin with whom she’s fallen in love.

Macdonald (“The Last King of Scotland,” “State of Play”) does a passable job of evoking post-apocalyptic atmosphere in “How I Live Now,” although the film suffers from uneven tone — is it a teen romance or wartime adventure? — and, ultimately, a regrettable lack of focus. The film’s supporting cast, playing Daisy’s plucky English cousins, is fine, and Mac­Kay makes a convincing object of adolescent desire, his shirtless chest becoming a repeating motif in Daisy’s dreams and psychic projections of her star-crossed love.

Primarily, though, “How I Live Now” is a showcase for Ronan to prove that she’s capable of more than pristine, angelic roles. Her eyes are still preternaturally blue, but she’s otherwise hidden her best qualities under glops of black makeup and coarsely dyed hair, her Irish brogue convincingly broadened into a Mid-Atlantic accent. Even when “How I Live Now” lags, she’s never less than completely compelling; her solemn, spiky watchfulness lends heft to an enterprise that otherwise seems fatally soft and reticent.

★ ★

Unrated. At Landmark’s E Street Cinema. Contains teen sexuality, brief profanity and wartime violence. 101 minutes.