Parts Per Billion tells the interwoven stories of three couples as a final, terrible war forces them to choose between survival and oblivion, hope and fear, life and death. LEN (Josh Hartnett) put his wife MIA (Rosario Dawson) through law school; now jobless, he's at home, putting up shelves, while she's out working, thinking about other men, testing her feelings. On the day the war starts, all that falls away - at least for a few hours - as they search for each other through the chaos.One of Mia's clients, ANDY (Frank Langella), though weighed down by guilt because of the part he played in causing the devastation, tries to persuade his wife, ESTHER (Gena Rowlands), that their love makes life worth living.Their grandson, ERIK (Penn Badgley), and his fiance, ANNA (Theresa Palmer) are so in love, so caught up in the intensity of their feelings, that the apocalypse seems like a distant offstage echo. Though she may be the only person alive who knew it was coming, Anna only has eyes for Erik, can only respond to him - he's the only thing that matters in her world.Inspired and sometimes blinded by their love, Len, Mia, Andy, Esther, Anna and Erik are as flawed and beautiful as any of the billions who are facing this human-made disaster.