Yuma is a young duelist who underachieves in both school and duels, although he dreams of becoming the world's greatest duelist. His traveling parents gave him a special key, which can unlock a door to another dimension, that he wears around his neck. In the second dimension, Yuma meets a mysterious spirit named Astral, who only Yuma can see and hear. Astral offers Yuma advice to help him in his duels. And Yuma helps Astral find his memories, which have been transformed into monster cards called Numbers. The duo must compete not only against the holders of the Numbers but also the Numbers hunters, who want to claim the Numbers cards for themselves in order to destroy Astral's homeland.