When Lady Mountjoy's diamond tiara goes missing, Polly, one of the maids, is accused of the theft. But Polly didn't do it. Discovered by the Baker Street Boys, out of a job and with nowhere to go, Polly begs them to help her - and so they find themselves with another mystery to solve. Wiggins hits upon a plan: why doesn't Queenie apply for the newly vacated job, infiltrate the house and see what she can find out? It seems there are certainly things to be discovered: (1) Lady Mountjoy is broke and in desperate need of money, (2) she has a wastrel brother living with her as a hanger-on, (3) both of them know Professor Moriarty.