Posing as amateur musicians, Professor Marcus and his gang rent rooms in the lopsided house of sweet but strict Mrs Wilberforce. The villains plot to involve her, unwittingly, in Marcus's brilliantly conceived heist job.

The police are left stumped but Mrs Wilberforce becomes wise to their ruse and Marcus concludes that there is only one way to keep the old lady quiet. With only her parrot, General Gordon, to help her, Mrs Wilberforce is alone with five desperate men. But who will be forced to face the music?