Anna (Rebecca Hall) and Will (Dan Stevens) were each other's first everythings: first kiss, first love, first relationship. Now years on, at Anna's thirtieth birthday party as Will is about to propose, the couple's best friend stands up and makes a drunken toast, suggesting that as their relationship is so functional, equitable and stable, it's threatening to their friends and perhaps they should sleep around before their inevitable marriage. The joke lingers and eventually Anna proposes that they try it, as a sexual experiment. As a couple, with complicity, they venture out of the purely monogamous boundaries of their relationship and in so doing are forced to evolve and grow up.