Set in Basildon, the film centres around Ben (played by The Dumping Ground), an 11-year-old boy who visits his paternal grandmother's (Julia McKenzie) house every Friday whilst his parents go out dancing. Ben dislikes the weekly visits as he finds Granny, who has an obsession with cabbage products, boring. After Ben finds a large box of jewellery, Granny reveals herself to have been an international jewel thief known as the 'Black Cat' involved in many high-profile heists during her youth but she tells Ben that she was never able to raid the Crown Jewels of the United Kingdom, kept in the Tower of London. This prompts Ben, who has a passion for plumbing, to devise a plan to raid the Tower. Granny agrees to go along with Ben's plans.