Professor Calculus's friend develops a blue-skinned orange that can grow on any kind of land and survive harsh weather (in the manner of Lue Gim Gong) and therefore solve world hunger. The Professor and his friends, however, run afoul of gangsters who also covet the fruit. The adventure takes them from their home in Marlinspike Hall (Moulinsart), a fictional mansion that is presumably in Belgium, to Spain, where Calculus and another scientist are kidnapped.
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