Dr. John Watson has just returned to London, after being wounded while serving with the military in Afghanistan. While looking for a place to live, a friend puts him in touch with Sherlock Holmes, and the two agree to share lodgings in Baker Street. Holmes proves an interesting if eccentric companion. When Holmes receives a message from the police, Watson learns that he is a private consulting detective, and soon finds himself assisting Holmes in investigating a murder case. Peter Cunningham has been found stabbed to death, with his fiancée standing over him holding a knife - but Scotland Yard's Inspector Lestrade is baffled by the lack of a motive for the crime.