Whilst his parents Linda and Mike spend every weekend ballroom dancing 11 year old Ben goes to stay with his granny, whom he initially finds boring. Then one day he finds a huge collection of jewels in a biscuit tin and discovers she was once a notorious burglar, the Black Cat, though she was never caught as she never tried to sell her spoils. Ben decides they should pull off the biggest heist of all, stealing the Crown Jewels from the Tower of London, using his intimate knowledge of plumbing to enter. Despite his parents' disastrous attempt to get him on the dance floor and the intrusive Neighbourhood Watch warden Parker the couple effect their daring plan. They do not succeed but are allowed to escape by a Royal personage, whom they happen to meet. Sadly Granny does not make it to Christmas but her legacy lives on - in the millions raised for charity by her jewels and the profound effect she has had on the Royal personage...