The series is set 150 years after humanity lost half of the planet due to a mysterious "black fog." To contain the "black fog," 3,000-meter-tall (about 10,000-feel-tall) walls were erected to split the world, and the half of the human population that survived now live in cities called "Arks." Shidou Enishi, one of the few Japanese survivors, runs a detective agency on the ninth ark. While pursuing a male suspect in a burglary, he learns of a massive conspiracy, one that may be tied to the Rivigarudo syndrome that is mutating its victims into something inhuman.

Kurono Kakuko, a middle school girl, has summoned a demon to grant her the power to wreck the world. However, the demon says it always seems like it's people who have no friends who ask for that sort of power, and he doesn't really want to do it anymore. Kurono insists that she's only alone because she doesn't need friends, and she ends up making a deal with a condition. If she can make friends that she can keep till graduation, then she'll be granted the power to wreck the world. For Kurono, this may end up being a very difficult task...

London in the 19th century is bustling as it prepares for The Great Exhibition. However, a Martian Tripod—a three-legged fighting machine—suddenly attacks the city. Amid the chaos, the orphan boy Oliver meets Alice, the daughter of a noble family. This is the story of the two, whose lives would never have had intersect under ordinary circumstances.

At the end of 1999, a few years after the terrorist attacks that killed 1999 people, Hisato Konoe, who has the memories of 99 people, receives a postcard from his sister, "I have sent the second most powerful assassin in the world." In order to protect Nanatose Miiku, Hisato must summon the strongest man in the world. (Source: Shogakukan, translated)