Anna and Will were each other’s first every-things: first kiss, first love, first and only relationship. Now, 10 years later, at Anna’s 30th birthday party as Will is about to propose, the couple’s best friend makes a drunken toast, suggesting that as their relationship is so stable and boring, 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 evolve.