Meet Allan Karlsson, soon-to-be centenary. The occasion calls for a celebratory party, with the mayor, the journalists, and all the old man's house patients and staff. But Allan has other plans... The book alternates between two storylines: the first, or "present" one, narrates Allan's escape from the institution and theft of a suitcase, while meeting new people and being chased by the owners of the suitcase, the criminal cartel Never Again, and by the police. The "past" storyline details the life of Allan, which meets lots of historycal figures and causes a lot of troubles.