As a dense yellow fog swirls through the streets of London, a deep melancholy has descended on Sherlock Holmes, who sits in a cocaine-induced haze at 221B Baker Street. His mood is only lifted by a visit from a beautiful but distressed young woman - Mary Morstan, whose father vanished ten years before.
Poirot, a Belgian refugee of the Great War, is settling in England near the home of Emily Inglethorp, who helped him to his new life. His friend Hastings arrives as a guest at her home. When the woman is killed, Poirot uses his detective skills to solve the mystery.
Holmes dramatically returns from the dead, or from being presumed dead. He and Watson reunite to take down a would-be assassin of Holmes, and murderer of Ronald Adair, Colonel Moran. With Moran in jail, Holmes can now safely return to London and resume his detective business.