Children's comedy based on the book by comedian David Walliams. Ben (Reece Buttery) dreads staying at his gran (Julia McKenzie)'s house when his mum and dad drop him off, expecting the usual boring games of scrabble and knitting. But on his latest visit, his gran reveals a fascinating secret - that she was once a renowned jewel thief known as the 'Black Cat' - and shatters Ben's image of his boring old relative. Together they set out on a whirlwind adventure to pull off the one robbery that always eluded her, breaking into the Tower of London and stealing the crown jewels.