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As a prophecy of doom unfolds on the peaceful land of Britannia, a purehearted boy sets out on a journey of discovery — and revenge.

Miaka, a junior high school student in Tokyo, has normal problems: fights with her mom, entrance exams, and deciding where she's going to go eat with her friends. But when she finds a mysterious book in the library, her life is changed forever when she and her best friend are transported inside!

Fourland Kingdom is an island nation in the North Atlantic. In this country, there are 52 X-Playing Cards, considered the origin of playing cards. A card with a unique supernatural ability can gift its abilities to a qualified person: the player. One day, the X-Playing Cards in government's custody were spread all over Fourland leading to troubles caused by the supernatural abilities of the players with cards across the country. The king secretly assigns Pinochle, one of the top corporations of the nation, to a mission, to collect the cards that are causing all the chaos in the country.

The story takes place in the year 1614. Two ninja clans, Tsubagakure of the Iga and Manjidani of Kouga, battle each other to determine which grandson of Tokugawa Ieyasu will become the next shogun. The deadly competition between 10 elite ninja from each clan unleashes a centuries-old hatred that threatens to destroy all hope for peace between them.

The story begins when high school student Kazuki Muto is killed one night saving a mysterious girl from a monster, only to wake up in his school dorm, believing it to have been a dream; however, he soon finds out that dream wasn't a dream at all when a giant serpentine monster attacks him and his sister. Tokiko Tsumura, the girl he saved, explains that the monster is a homunculus. Kazuki had been attacked and killed by it when he was rescuing her; however, she, feeling responsible for him, revived him by placing a Kakugane medallion in his chest, serving as a replacement heart.

Sato is a high school boy in love with his best friend Hime, an unearthly beauty from another realm. So when she moves back to her home world to get married, Sato doesn’t think twice—he follows her and crashes the wedding. Then, after a kiss from Hime, he suddenly becomes the new groom! But here, Hime is a Ring Princess and her husband is destined to be the Ring King: a hero of immense power.

Kasugano Haruka and his sister Sora have lost both their parents in an accident, and with them all their support. They decide to move out of the city to the rural town where they once spent summers with their late grandfather. At first everything seems familiar and peaceful, but changes come as Haruka starts to remember things from his youth.

After failing his college entrance exams, 18 year-old Tsukasa Mizugaki is offered a position at the renowned SAI Corporation, known for its production and management of Giftia, androids that possess human emotions. Tsukasa’s position is in the terminal service department where the main job is to recover Giftias that are close to their expiration, a graveyard department in every sense. To make matters worse, Tsukasa is ordered to work with Isla, a female Giftia who is never given any responsibility other than serving tea to co-workers.

Shy Natsu awakens as part of a group chosen to ensure the survival of humanity. Together, they have to survive on a changed Earth.