''Untogether'' follows the affair between Andrea (Jemima Kirke), a former teen prodigy turned heroin addict who is trying to be a writer now that she's sober, and Nick (Dornan), a writer who's found success with his memoir of wartime bravery, which sees him showered in wealth and women. Meanwhile, Andrea's little sister, Tara (Lola Kirke), finds her solid relationship with her older boyfriend, Martin (Mendelsohn), shaken when she is drawn to a charismatic rabbi, David (Crystal), with an even larger age gap.