On his one-hundredth birthday, Allan Karlsson, tired of the unwanted celebrations, quietly climbs out of his nursing home window and makes his getaway. This sparks a picaresque journey involving criminals, several murders, a suitcase full of cash, and incompetent police. As his current escapades unfold, readers discover Allan's remarkable past, where he helped create the atom bomb, befriended world leaders, and participated in many key events of the twentieth century. His life story is as improbable and adventurous as his present-day escape.