30th November 1803: a corvette sets sail from the port of La Coruña to much cheering and applause. On board are twenty-two orphaned children whose mission consists of taking the recently discovered smallpox vaccine to the territories of Ultramar. They are accompanied by Isabel Zendal, whose task is to look after them. Lead by Dr Francisco Xavier Balmis and his assistant Josep Salvany, the heroes of the expedition will survive storms and shipwrecks, and confront the opposition of the clergy, the corruption of officials and greed. If this adventure ultimately became the greatest humanitarian feat in human history, it is due not only to the courage of those children who found themselves dispatched to save the lives of so many people, but also to the bravery of the two leaders, fearless men who fought for the love of the only woman on board.