Anna (Rebecca Hall) and Will (Dan Stevens), were each other's first every-things: first kiss, first love, first and only relationship. Now, 10 years in, at Anna's 30th birthday party, as Will is about to propose, the couple's best friend makes a drunken toast, suggesting that they should sleep around before their inevitable marriage. The joke lands like a lead balloon, but the thought lingers until Anna proposes that they try opening their relationship--as a sexual experiment. Together, they venture out of the purely monogamous boundaries of their relationship and, along the way, evolve.