From the Introduction page

Greetings, investigators! Welcome to Baker Street: Roleplaying in the World of Sherlock Holmes.

The year is 1891, and Dr. Watson needs your help. Holmes is missing and cases are pouring into Baker Street. It is up to you to select a Victorian profession and put your brain power to work against Sherlock Holmes's greatest foes.

In this book you will find rules for making your own investigator, investigating mysteries, fighting criminals, and creating and running your own Baker Street adventures.