A mysterious illness is afflicting members of an archaeological expedition recently returned from the Andes, where they had unearthed the tomb of the Inca, Rascar Capac.

One by one, the expedition members fall into a mysterious coma. The only clue is shards of crystal found near each victim, which are fragments of shattered crystal balls. Concerned, Tintin, Captain Haddock and Professor Calculus go to stay with Calculus's old friend, and expedition member, the ebullient Professor Tarragon, who is keeping Rascar Capac's mummy in his house and is being tightly guarded by the police against any attack. The mummy is evaporated when a lightning storm sends a ball of fire down the chimney, and, after each being visited in their nightmares by the mummy, the three wake to find Tarragon comatose, with the telltale shards of crystal by his bed. The attacker bypassed the police watch by coming down the chimney. The police shoot the attacker as he flees, but fail to capture him.

Tarragon later wakes up but screams about mysterious figures attacking him. Tintin later visits a hospital where all the other stricken explorers go through the same horrors at a precise time of day.

The plot thickens even further, however, when Calculus, taking a stroll around Professor Tarragon's house, discovers a striking gold bracelet, puts it on (remarking on how nicely it goes with his coat), and then mysteriously disappears...