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At Kat Whitney's request, Dr Watson goes to an opium den to bring home her husband, Isa Whitney. Unexpectedly, he meets Sherlock Holmes, who pretends to be a drug addict in order to find out the circumstances of Mr Neville St Clair's disappearance. At the hotel where St Clair was last seen, the police detained the only occupant, Hugh Boone, a colourful professional beggar with an impressive split lip. After a night's reflection, Holmes goes to the prison and unmasks Hugh Boone by washing off his make-up. The beggar is recognised as the missing St Clair.