Sherlock Holmes and Dr Watson come to the help of the landlady Mrs Warren. She has rented out a room to a man who has offered to pay twice the asking price, as long as she meets his requirements. The deal settled, the man left, only coming back when everyone was asleep. She leaves him meals and the daily paper on a chair by his door, where the lodger also leaves her a note if he needs anything. Since that first day she has not seen him.
"The Adventure of the Red Circle" is part of "His Last Bow".

Sir Arthur Conan Doyle (1859-1930) was born in Scotland and studied medicine at the University of Edinburgh. After his studies, he worked as a ship’s surgeon on various boats. During the Second Boer War, he was an army doctor in South Africa. When he came back to the United Kingdom, he opened his own practice and started writing crime books. He is best known for his thrilling stories about the adventures of Sherlock Holmes. He published four novels and more than 50 short-stories starring the detective and Dr Watson, and they play an important role in the history of crime fiction. Other than the Sherlock Holmes series, Doyle wrote around thirty more books, in genres such as science-fiction, fantasy, historical novels, but also poetry, plays, and non-fiction.