It all starts on the one-hundredth birthday of Allan Karlsson. Sitting quietly in his room in an old people's home, he is waiting for the party he-never-wanted-anyway to begin. The mayor is going to be there. The press is going to be there. But, as it turns out, Allan is not... Slowly but surely Allan climbs out of his bedroom window, into the flowerbed (in his slippers) and makes his getaway. And so begins his picaresque and unlikely journey involving criminals, several murders, a suitcase full of cash, and incompetent police. As his escapades unfold, we learn something of Allan's earlier life in which - remarkably - he helped to make the atom bomb, became friends with American presidents, Russian tyrants, and Chinese leaders, and was a participant behind the scenes in many key events of the twentieth century.