Marcus rents rooms from a sweet, eccentric old lady, Mrs. Wilberforce, in her crooked London house. The professor and his co-conspirators, blowhard Major Courtney, creepily suave Louis, chubby Harry, and muscleman One-Round, pose as an unlikely string quartet using the rooms for rehearsal. Dodging Mrs. Wilberforce's constant interruptions, the hoods hit upon the idea to use her in the daring daylight robbery. When the old girl discovers the truth, Marcus and company cannot persuade her to stay buttoned up about it and thus decide to do her in.