After years of marriage, Joy and Alan Richards find themselves in a sexual rut. Therapist Joy is trying to find a way to keep her spark with her husband, following a cycling accident which has put their sex life on hold. Suddenly Joy and Alan find themselves drawn to new people, and both are tempted to make the ultimate indiscretion. With their marriage in crisis Joy comes up with an audacious, unorthodox, but potentially brilliant proposal that will have huge ramifications for the future.
Alan has sex with Claire whilst Joy does the same with social worker Marvin. Alan is aroused to hear his wife's account of her evening but the pair are interrupted by daughter Laura, dead drunk after her boyfriend left her. After confessing all to her colleague Angela Joy decides to tell Marvin about her arrangement with Alan but he is disgusted and walks out, leading to her making a bar pick-up, Marc. However they are observed by Laura and her new boyfriend Jason - who just happens to be one of Laura's clients.
Joy and Alan explain their arrangement to Laura and her siblings Naomi and Tom - who seems upset by the news. Nonetheless Joy prepares to meet for sex with Marc but bumps into old flame Lawrence whilst keeping from Laura Jason's troubled past. Joy and Alan finally get round to having sex but this intensifies Alan's feelings for Claire.
Joy and Alan go on a double date with Claire and Marc which invigorates the Richards' sex life though Alan is forced to confess the arrangement to his head teacher Janet. Joy learns that Lawrence's father has died and offers comfort but rejects his advances. Meanwhile Tom gets closer to Michelle and Naomi shares a moment with Rita, home-making wife of Alan's colleague Neil.
Joy and Alan face up to the seismic changes in their lives and try to decide if the choices they have made are the right ones. After a tense encounter with Claire, a lost and lonely Joy takes a risk on love and goes on a bold journey. Tom's new relationship progresses in a surprising way, and Laura figures out exactly what, and who, she wants for herself. And Joy makes one final decision about the future.