Professor Calculus has just written a book called "The Earth is Starving". He appears on television to appeal to the scientists of the world to work towards a solution to the hunger problem. Soon afterwards, he receives a package from a Spanish professor, Antenor Zalamea, which contains a blue orange that can be grown in desert soil. Calculus travels to Spain, but a criminal emir who wants the discovery for himself kidnaps both professors. Tintin and Captain Haddock come to the rescue with the help of some local children.
Source : IMDb