When Runningbrook Tennis Club's owner Frederick Greenwood, new beau of Lu's mother Genevieve, receives a death threat, he hires Frank and a reluctant Lu to find the culprit and go undercover as potential new members. Lu, preoccupied with revision for her impending PI exam, takes an immediate dislike to Frederick, and with staff security codes incriminating surly gardener Adam Oldknow, bitter ex-tennis pro Rose Lin, and ladies' man club captain Orlan Rowlands, our detectives have their work cut out for them. But when the club's mascot costume is found stabbed with a ransom note demanding £100,000, and Orlan is poisoned during the club's Battle of the Sexes match, the case takes a turn for the deadly. Will the blackmailer be discovered, and at what cost for Lu? Will she make her exam in time and finally qualify as a PI?