Post-apocalyptic drama 'How I Live Now' slogs along

“There's going to be World War III!” pipes up a little girl named Piper in Kevin Macdonald's cinematic adaptation of Meg Rosoff's novel about teen angst and yearning amid a world gone seriously haywire.

Up to this point in “How I Live Now” (not one of the catchiest titles to grace a marquee), the story has followed Daisy, a transplanted bratty American teen played by the electrifying Irish actress Saoirse Ronan.

She has come to live with her distant relatives in the rural countryside of Great Britain for a summer. Filled with attitude and punky spunk, Daisy disdains life with little Piper (Harley Bird), 14-year-old Issac (Tom Holland), older Edmund (George MacKay) and their mom (Anna Chancellor).

Mom heads off to Geneva to work on some kind of peace talks, leaving the kids behind to bond in an idyllic rustic retreat where Daisy blossoms and begins to have feelings for the redheaded Eddie, an “animal-whisperer” who apparently can talk to hawks, cattle and girls.

Then comes the distant rumble. The hurricane-force winds. The eerie snow flakes in summer. London has been nuked, throwing the country into martial law. The kids get rounded up and shipped off to separate labor camps.

We don't know who the enemy are and what goes on outside of Daisy's world view. In a way, this becomes a major asset to “How I Live Now,” simply because the not-knowing becomes a constant source of disequilibrium for us.

Still, that plus Ronan's piercing, intelligent blue eyes don't make up for Macdonald's mostly inert post-apocalyptic drama that puts Daisy and Piper on a “Lassie Come Home” mission to return to their homestead, because Daisy dreams that her beloved Eddie will be there.

Eddie's touch of supernatural ability ultimately proves pointless, leaving Ronan's highly watchable, empathetic performance to carry us through a sloggy nightmare fraught with a mixture of mild suspense and plodding inanition.

“How I Live Now” opens at the South Barrington 30 and the River East 21 in Chicago. Rated R for language, sexual situations and violence. 100 minutes. ★ ★ ½