International terrorists attempt to kidnap a wealthy couple's child. Their plan comes unstuck when a deadly Black Mamba, sent by mistake instead of a harmless snake, escapes and the terrorists and several hostages are trapped in the boy's London home.
In the late 1970s, Cockney crime boss Harold Shand, a gangster trying to become a legitimate property mogul, has big plans to get the American Mafia to bankroll his transformation of a derelict area of London into the possible venue for a future Olympic Games. However, a series of bombings targets his empire on the very weekend the Americans are in town. Shand is convinced there is a traitor in his organization, and sets out to eliminate the rat in typically ruthless fashion.
Unscrupulous boxing promoters, violent bookmakers, a Russian gangster, incompetent amateur robbers and supposedly Jewish jewelers fight to track down a priceless stolen diamond.
Dr Trevelyan calls on Holmes on behalf of his friend Blessington, after a Russian father and son vanish during a consultation. Blessington knows of no one who wishes him ill, but he is found hanged the next day. Holmes investigates, with advice from Watson.