It all starts with Snowy's picking in garbage and finding a can with crab picture printed on it. Later that day he meets Thompson and Thomson and finds a piece of paper with the same crab picture in their possession. Tintin finds the word Karaboudjan at the back of the paper. He learns from the Thompson Twins that the drowned man in whose pocket they found the paper was a man from the ship called Karaboudjan. So he pays a visit to the ship and the first mate Allan senses that Tintin is upto something and tries to kill him. While cornered in a cabin, Tintin finds out the crab tins are full of Opium. Tintin finds Captain Haddock in the ship who is an honest man and easily satisfied with Whisky. So First Mate Allan just keeps him under his control with Whisky and uses the cargo to smuggle Opium. Tintin takes the Captain with him when he escapes from the ship.