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.
an extended description!