The "Dagger of Batu Khan", a goldsmith's work from the 10th century that was thought to be lost, is discovered in a bricked-up cellar vault. Sebastian, whose father is the head curator at the "Green Vault" in Dresden, and his friends are allowed to help prepare the museum's exhibition of the dagger. They accidentally learn that the dagger is to be stolen and their lives are put in danger when they thwart the planned theft.

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