Kate Whitney the wife of Isa Whitney seeks Dr.Watson's help to rescue her husband an opium addict who had not returned home for more than two days. Kate tells Watson that her husband was in the habit of frequenting an opium den called the Bar of Gold in Upper Swandam Lane along the banks of the river Thames.
Soon Watson reaches the opium den in Swandam Lane in one of the filthiest localities in London. Watson remarks that the den was dark and hazy with opium smoke and had tiers of berths where each addict lay sprawled about puffing his pipe of opium.
Upper Swandam Lane is a vile alley lurking behind the high wharves which line the north side of the river to the east of London Bridge. Between a slop-shop and a gin-shop, approached by a steep flight of steps leading down to a black gap like the mouth of a cave, I found the den of which I was in search. Ordering my cab to wait, I passed down the steps, worn hollow in the centre by the ceaseless tread of drunken feet; and by the light of a flickering oil-lamp above the door I found the latch and made my way into a long, low room, thick and heavy with the brown opium smoke, and terraced with wooden berths, like the forecastle of an emigrant ship.
Through the gloom one could dimly catch a glimpse of bodies lying in strange fantastic poses, bowed shoulders, bent knees, heads thrown back, and chins pointing upward, with here and there a dark, lack-lustre eye turned upon the newcomer. Out of the black shadows there glimmered little red circles of light, now bright, now faint, as the burning poison waxed or waned in the bowls of the metal pipes. The most lay silent, but some muttered to themselves, and others talked together in a strange, low, monotonous voice, their conversation coming in gushes, and then suddenly tailing off into silence, each mumbling out his own thoughts and paying little heed to the words of his neighbour. At the farther end was a small brazier of burning charcoal, beside which on a three-legged wooden stool there sat a tall, thin old man, with his jaw resting upon his two fists, and his elbows upon his knees, staring into the fire.
After Watson had successfully located Isa Whitney amongst the addicts and was leading him out of the den, he felt someone pulling his dress to his shock and surprise he discovers that it is none other than Holmes himself. Holmes after heartily laughing at his friend's astonishment explains to him that he has come to this wretched place to trace out a missing person by the name of Neville St. Clair.
Then, glancing quickly round, he straightened himself out and burst into a hearty fit of laughter.
"I suppose, Watson," said he, that you imagine that I have added opium-smoking to cocaine injections, and all the other little weaknesses on which you have favoured me with your medical views."
"I was certainly surprised to find you there."
"But not more so than I to find you."
"I came to find a friend."
And I to find an enemy."
Yes; one of my natural enemies, or, shall I say, my natural prey. Briefly, Watson, I am in the midst of a very remarkable inquiry, and I have hoped to find a clue in the incoherent ramblings of these sots,
Holmes pretends to be an opium addict so that he can overhear secretly the drug induced loose talk of the addicts and hopefully gain the information that he had wanted.