Two youths, Shunta Mogami of Earth and Yoku Albatrosa of Valgarde are transported into a mysterious dimension called the Spirits World because they are heroes chosen by Exeseed and Gale-Phoenix respectively, two of the Twelve God-Kings.

Two youths, Shunta Mogami of Earth and Yoku Albatrosa of Valgarde are transported into a mysterious dimension called the Spirits World because they are heroes chosen by Exeseed and Gale-Phoenix respectively, two of the Twelve God-Kings who are all-powerful guardian deities of this world. With the help of the benevolent priestess Eto and her aide Kinoto, Shunta and Yoku must prevent the Battlers of Darkness, led by the cruel despot Tatsumi, from finding all twelve of the God-Kings and using their powers to release the Evil God-King, the embodiment of chaos and evil who once threatened to engulf Spirits World in eternal darkness and was sealed long ago by the God-Kings and their chosen heroes. On the way, they would meet new allies and encounter more enemies in their quest for searching the twelve God-Kings and placing them in the right hands.—Battle Spirits Wiki

Shunta Mogami loves the Battle Spirits game. One day he is transported by a light from a Battle Spirits card to Spirits World, the origin of all Battle Spirits. He meets a girl named Eto, who claims to have summoned him to Spirits World in order to prevent its destruction at the hands of the revived Evil God-King, who once plotted the destruction of Spirits World before being defeated and sealed away. Together with the confident, green-deck-wielding Yoku Albatrosa, who also came from another world, Shunta sets out to find the twelve God-Kings whose power originally sealed the Evil God-King.—Anime News Network