Ronan plays the part of Daisy, an American teen caught in the midst of a war taking place in England in the near future.

Daisy is sent to England to live with her aunt and cousins in the English countryside for the summer. She exists with her cousins in a sort of dreamland and falls madly in love with Eddie, played by George MacKay.

All is well until their perfect summer is blown apart when an unnamed force invades England.

The teens along with the younger children are left in isolation and must fend for themselves.

Daisy is faced with the task of reuniting the family after soldiers arrive at the farm and violently tear the teens apart. She must embark on a terrifying journey, one she hopes will lead her back to Eddie.

Ronan recently starred in Andrew Niccols' science fiction romantic film, The Host, based on the novel by Stephenie Meyer. She will soon take on the lead role in Wes Anderson's, The Grand Budapest Hotel.

How I Live Now is based on the 2006 novel of the same name by Meg Rosoff.

Watch the trailer for the film now, and catch the full thing from October 4.