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.
Stevens and Hall are always interesting to watch. (In fact, you should watch him in "The Guest" and her in "The Awakening," if you want to see two excellent, lesser-known films.)
While it deals with heavy relationship issues including the validity of monogamy, it manages an easy, seemingly effortless humor that seduces the audience while simultaneously breaking filmgoers' hearts.