When a teenage pyromaniac terrorizes a neighborhood, the police are helpless as frightened victims refuse to testify against him. Enter Hetty, posing as the Irish sister-in-law of one of the residents. Her plan: infiltrate the local sewing circle and get the lowdown on Lenny the firebug and his gang.

Hetty sends Geoffrey undercover when it comes to light that the owner of a local gym and fitness centre may be supplying people with dangerous anabolic steroids.