Professor Marcus and his criminal friends are planning a robbery of an armored truck. To carry out their plan, the shady gang rents a room from the elderly and seemingly somewhat confused Mrs. Wilberforce, right next to King's Cross Station. As a cover, they pose as a string quintet, even though none of them can read a single note – the melodies come only from a gramophone. The robbery is a success, but Mrs. Wilberforce is onto them – who would have thought that the biggest obstacle in their heist would be their shrewd landlady...