In the novel Nemesis, Broadribb & Schuster is the law firm handling the personal (as opposed to the business) affairs of Mr Rafiel. After his death, they were responsible for executing his will. The firm has its office in Bloomsbury.
At the beginning of the story, acting according to Mr Rafiel's wishes, the partners of the firm, Mr Broadribb and Mr Schuster, met Miss Marple and handed her a letter from Mr Rafiel. In it, Mr Rafiel asked Miss Marple to undertake an investigation into a crime, without specifying what crime it is, but mentioning the code word "Nemesis". Neither Broadribb nor Schuster had any idea what the investigation was to be about either. Broadribb did have a set of sealed instructions from Rafiel to be opened only under a set of conditions.
In BBC's 1987 adaptation of the novel, the firm had a more active role as they were instructed to render Miss Marple whatever assistance necessary. Through the course of the plot, they performed various errands for her, such as forwarding documents and tracing a parcel.