From MangaHelpers:

Kouya Mirai used to play the Duel Masters card game as a kid, but now that he's 15, he's long since put that stuff behind him. He even buried his old cards in a time capsule. However, when his friend Shiratori Manami drags him to a 'part-time job,' he finds himself once again playing Duel Masters in a card shop. With the feelings of nostalgia this brings up, he and Manami visit their old elementary school that night-- only to find it full of people searching for his time capsule! When Mirai recovers his time capsule, he finds that what is inside is bigger than just a card game, it is something that appears to connect the Duel Masters monsters to reality! [tethysdust]