According to her own complex moral calculations, Katie Carr has earned her affair. She's a doctor, after all, and doctors are decent people, and on top of that, her husband David is the self-styled Angriest Man in Holloway. But when David suddenly becomes good - properly, maddeningly, give-away-all-his-money good - Katie's sums no longer add up, and she is forced to ask herself some very hard questions.