The plane Tintin and Haddock are using to escape crashes on Sahara and they got saved by the Lieutenant Delcourt's people. After several days journey they reach Bagghar, where they learn about Omar Ben Salaad who is the biggest trader in the city. The Thompson Twins charges him directly and Omar gets very angry. Just as he was throwing the Twins out Tintin and Captain finds Omar's celler full of opium in crab tins and Omar with a necklace with two gold crab claws. The story ends with the whole gang getting captured by the police, even First Mate Allan who tries to escape. Captain gives a speech about how a sailor's greatest enemy is Alchohol and gets ill by drinking a glass of water.

Tintin is not the only one who knows the secret of the parchment hidden inside the ship's mast. The Byrd Brothers, antique dealers, also find a copy of the model ship Unicorn in their basement. They also start looking for the other two parchments, and they are not really the friendly kind. When their agent, Barnaby, tries to warn Tintin about the Byrd Brothers they try to kill him and also kidnap Tintin to find out about the parchments that they are missing. At last the parchments are recovered and the mystery solved.