In the advanced, futuristic metropolis of Heartland, a boy named Yuuma Tsukumo chases a dream shared by kids his age: becoming the greatest duelist. However, his poor dueling skills make this goal seem out of reach. When the school bully, Ryouga "Shark" Kamishiro, breaks the key given to him by his father into two halves, Yuuma is set on a path toward his ambition.
Yuuma keeps one piece of the key and starts a duel with Shark, but quickly finds his inexperience leaves him outmatched. Suddenly, the key mends itself, and the "Door of Destiny" appears before him. Unlocking it with the key, Yuuma meets an alien named Astral—a being visible only to him.
Confused, Yuuma learns that his memories have been split into 100 "Number Cards," which he must recover. However, others are also hunting for them. Overwhelmed by these discoveries, Yuuma, with Astral's help, must protect the world from the dangers looming over Heartland City, no matter what divine interference might arise.