The BBC drama airs its gripping final episode tonight. Based on Louise Doughty’s bestselling novel, Apple Tree Yard follows a highly successful scientist named Yvonne Carmichael (Emily Watson), who is married with children, as she embarks on a secret steamy affair with disastrous consequences. In this episode, the murder trial continues, with Costley's lawyer Bonnard basing her defence on evidence that Costley suffers from a personality disorder which would have impeded his ability to react normally during an altercation with Selway. The prosecution attack this claim. When Yvonne finally takes to the stand, she sticks firmly to her story and confidently holds her own.