One day, Muto Kazuki, a high school boy, saves a girl being assaulted by a big monster, but he is stabbed to death by the monster. However, the next morning, he finds he is alive. He wanders it was just a dream, but he meets the monster again, and he realizes his death is real. He and his sister get into a scrape, but suddenly, a girl in the dream appears. She explains him that the monster is a Homunculus made by alchemy. She also tells him that there is Kakugane in his chest in place of the heart. It is an alloy made by alchemy, too, and it is the only weapon to fight against Homunculus. Using the power of Kurogane, he beats the monster and saves his sister.
Kazuki Muto. He died trying to save a girl. He was given a second chance by that girl and because of that he gained an Arms Alchemy weapon capable of killing the being that killed him. He also seems like a natural-born fighter.
The girl that Kazuki though he was saving. Her name is Tokiko Tsumura. And she is an alchemic warrior! Her Arms Alchemy is call Valkyrie Skirt. She gave Kazuki the Arms Alchemy Kakugane to save him because of his earlier sacrifice.
Allen Walker is a young Exorcist who has a cursed eye that can see the suffering of the Akumas created by The Millenium Earl. He travels to England to the Headquarters of the Exorcists where his Master has sent him to meet with the other Exorcists. The battle with the Millenium Earl and the Noah Family vs. the Exorcists intensifies when some of the best Exorcists begin to get killed by hoards of Akuma. Japan takes center stage in this end of 19th Century plot in order to not let the entire human race be turned into Akumas and the Exorcists have the power of “Innocence” that was given to them by God on their side.