Having arrived for the summer with his uncle and aunt, 12-year-old Henry York was sure that nothing interesting should be expected from a provincial town in Kansas. However, on the very first night that the boy spent in the attic room, strange things began to happen to him. After this, Henry discovered that on one of the walls in his room, under a thick layer of plaster, there were compass-like handles and 99 small doors, behind each of which a different world was hidden. Together with his cousin Henrietta, the boy decides to find out more about the doors and travel through unknown worlds.