11岁的本恩觉得自己80多岁的奶奶只会嚼薄荷糖、玩拼字游戏，身上全是臭臭的圆白菜味儿，他讨厌和奶奶待在一起。但是有一天，奶奶要带他去冒险，本恩开始研究地下管道，练习游泳，学习各种逃生技巧……当他和奶奶一起就要完成冒险计划时，英国女王突然出现在他们面前，本恩面对的是惊喜还是灾难？ 　　本剧根据David Williams同名儿童书改编，Miranda Hart, Robbie Williams倾情客串，一同为大家献上欢乐的圣诞大餐！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.