Dangerfield becomes involved in the case of his long-time patient, businessman Patrick Hooper, when he leaves his wife Margaret to embark on a new relationship with attractive Caroline White. However, Hooper claims to Dangerfield that Caroline has been receiving threatening letters from an allegedly unhinged Margaret and as a result, Paul asks a reluctant Joanna to try and find out whether Margaret is showing signs of mental instability. Later, events take a tragic turn as Hooper's mistress Caroline is killed. DI Ken Jackson and his team find themselves in the middle of a sinister family triangle as both Margaret and the Hoopers' son Thomas end up as prime suspects in Caroline's murder - however, all is not what it appears and with the help of forensics expert Terri Morgan, the police discover that the blame lies in an unexpected quarter.