Suske and Wiske, Lambik, and Aunt Sidonia are staying at the Ganzehoeve estate. During a walk, Suske and Wiske find a wooden box containing a black stone. They give the stone to Sidonia, who sells it to the baron. The next day, Sidonia is lying in bed, seriously ill. "Anyone who tries to enrich themselves with the stone will die before the next full moon unless the stone is returned to the alchemist." Suske and Wiske must return the diamond to the alchemist at all costs. They enlist the help of Professor Barabas and use the time machine to flash back to the 15th century. This marks the beginning of an exciting quest.

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