It’s breathtaking how many tones and themes are skillfully juggled in “How I Live Now.” Based on Meg Rosoff’s prizewinning young-adult novel, it’s much like the first teenage love it portrays: by turns high-spirited, visceral, tragic and absurd, hitting with the impact of a meteorite.

The film opens as a spiky character drama as we meet Daisy (Saoirse Ronan), a rude, pugnacious New Yorker in tattered goth leggings, en route to rural England. Amanda Palmer’s punk anthem “Do It With a Rock Star” is pounding in her earphones, and self-help catchphrases are echoing through her pouty little mind. She’s been shipped off to spend the summer with her four young step-cousins; Daisy regards their un-ironic friendliness as if it were something from the barnyard stuck to her shoe. Her attitude mellows when she encounters Edmond (George MacKay), the crew’s handsome eldest brother.

Daisy begins to bloom a bit as the kids chase cows in emerald-green fields and swim in the nearby river. Over the course of 45 minutes, she warms to Edmond. Standing at the cusp of adulthood, she sees a better future ahead. Like Peter Pan’s Lost Boys, the clan fend for themselves as mother, an important diplomatic, travels to Geneva for meetings about something to do with projected mega-deaths. “There’s going to be World War III,” declares youngest sister Piper with innocent nonchalance.

Director Kevin Macdonald (“The Last King of Scotland”) edges us almost imperceptibly into wartime drama and thoughtful science fiction. He never strays from the limited perspective a young person would have of international events, yet he provides suspenseful hints of the global back story. The shiny next-generation security technology at the airport’s passport control implies a security state even more vigilant than today’s. Is the TV news report of a fiery bombing in Paris mere happenstance or the spark to a global powder keg? The military aircraft fleetingly glimpsed overhead carry a foreboding charge. The fact that information is scarce adds to the anxiety.

The children’s carefree summer, shot with handheld cameras and rambunctious energy, passes with a sudden gust of breeze. A flurry of ash, mimicking snow, blankets the farmstead. That’s what’s left of London. Martial law assigns the boys to one labor camp, the girls to another. Daisy will risk anything to be reunited with Edmond. Young Piper in tow, she makes her way across the war-torn wilderness toward the old farm. Our teen romantic drama has evolved into a harrowingly realistic dystopian thriller with terrorist attacks and mounds of corpses.

Ronan, who has matured into a classic screen beauty, skillfully illuminates Daisy’s complicated character arc. She carries us with her as she evolves from sullen prefab rebel to heart-struck romantic to hardened survivor. Even in the most extreme moments, her performance retains a solid link to realism. Daisy is not a glorious Katniss Everdeen-style liberator, but a small-scale heroine whose first passionate romance gives her the will to survive.

The film remains rooted, too. Macdonald began as a documentary filmmaker before moving on to features. He creates something here that’s more than a commercial contraption catering to teen wish fulfillment. With its intelligent energy and standout lead performance, “How I Live Now” takes its place alongside this season’s strong gallery of survival-against-all-odds movies.