The film is about Allan Karlsson who decides to escape his boring everyday life in a nursing home as a gift for his 100th birthday.
 
He climbs out of a window and a series of events embarks him on a hilarious and entirely unexpected journey, involving a gang of criminals, murderers, a suitcase stuffed with cash, an elephant and an incompetent policeman. It would be the adventure of a lifetime for anyone else, but to Allan it is just ordinary life. Not only has he witnessed some of the most important events of the twentieth century, he has actually played a key role in them. Although Allan is completely uninterested in politics and religion, he seems to have affected most of the last century's major world events.
 
We also get to follow Allan's previous escapades. How he, among other things helped to invent the atomic bomb, and became good friends with everyone from U.S. presidents to Russian tyrants. For a hundred years Allan Karlsson did rattle the world, and now he's on the loose again.