It’s only teenage wasteland, as Pete Townshend once sang. “How I Live Now” is a meticulously observed, rapturously directed account of World War III and its aftermath as seen from the point of view of a spoiled young woman. The movie’s pretty fascinating before it goes bonkers.
There is talent involved. Director Kevin Macdonald made “The Last King of Scotland” and specializes in hallucinatory hyperrealism. Saoirse Ronan, once the spooky little tattletale of “Atonement,” has added this role to the roster of damaged adolescents she’s collecting on her way to adulthood: the kid assassins of “Hanna” and “Violet & Daisy,” the regretful vampire of “Byzantium,” the alienated split personality of “The Host.”