Miss Mallard arrives in foggy London to visit her nephew, Willard, who is in town working on a big international spy ring case. Little do they both know that an innocent ride in a taxi and their kind attempts to return a lost birthday cake with its scripted birthday message to the mysterious Milly will lead Miss Mallard and Willard on a search through London that ends with them solving the very case Willard is working on.

几个月里，欧洲各国都连续发生机密文件被盗事件。维乐与他的上司光头总探长受命代表瑞士警方到英国伦敦协助苏格兰场调查这起跨国间谍案。梅小姐正巧也来到伦敦。在她前往苏格兰场探望维乐的途中，一只公鸭冲过来将一只温斯顿与克里斯糕饼店产的生日蛋糕塞给梅小姐就跑开厂。而维乐因为向姑妈大谈特谈间谍案的情况被光头总探长解了职，只能跟着姑妈四处寻找蛋糕的主人米莉。每到一处，梅小姐都闻到一股杏仁味儿，随后就遇到生命危险。所有蛋糕的定户都拿到了自己的蛋糕，那这个蛋糕会是谁定的呢?维乐刚想吃了它却摔了一跤，让梅小姐意外地发现了一个微型胶卷，从而找到了间谍组织的头跤。维乐也因此恢复了职务，赢得了苏格兰场的好评。