An action series that meshes many different genre's into one.
Kazuki Muto decides to take a stroll up to the haunted hills one night. While up there he sees a young women passing by, problem is she is about to get attacked by unknown monster. Kazuki with no thought for his own life jumps in front of the monster attacking this young women. However in trying to save this young women he is stabbed through the heart and gets killed. Little did he know his efforts weren't needed. The young women was Tokiko Tsumura an Alchemy Warrior who was trying to lure out this monster from hiding. Feeling indebted for this young many completing throwing his life away for a complete stranger, Tokiko inserts a Kakugane into his heart bringing him back to life. He wakes up the next morning thinking that everything that has just passed was nothing but a dream. While late at school the next night the same Homunuculus attacks him, but he is saved by the very same women he tried to save the night before. She explains everything that happened and tells him to go home and forget about this world. However he can't help but want to protect everyone so he runs to the Homunuculus nest and saves Tokiko as she is about to get attacked by the main Homunuculus there, by forming his Kakugane(Buso Renkin) into a lance called Sunlight Heart.
Koushaku Chouno - Becoming Super Human
Kazuki decides he wants to stay in this dangerous world to try to help and protect everyone he can. He is then recruited into the Alchemy Regiment, and is order along side Tokiko to investigate the growing number of Homunculus in Gensei City. The series then takes you into several steps of Kazuki fighting Homunculus and growing stronger and learning more about his Buso Renkin. However all is not well as Tokiko is attacked, and is giving one week left to be human. If they don't find the creator of the Homunculus in Gensei City and get the antidote then Tokiko is going to be turned into a homunculus. They figure out the creator of the Homunculus is a student at his school known as Koushaku Chouno. A senior of whom no one ever heard about. He was dying from an illness and was trying to become a superhuman before he died. Kazuki wants nothing more than to try to protect Chouno and those around him. However he fails and Chouno becomes a super human and Kazuki is forced to kill him in order to save Tokiko.
Chouno however is not dead(He also takes a liking to Kazuki who is the only person in the series to ever treat him as a human), as he is revived by his great-great-grandfather, Dr. Butterfly who is the leader of an organization of human-type Homunculus, known as the LXE. This organization was formed a 100 years in the past by Dr. Butterfly. He was given the secrets of becoming a human-type Homunculus in exchange for helping a traitor of the Alchemy Regent, named Victor. Since the Homunculus in this group are human-type the have the ability to use the Buso Renkin at there will.
Kazuki, Tokiko and there new commander, Bravo are sent to investigate the growing threat of LXE and to stop the revival of victor. Kazuki is forced to fight two of his classmates, who were dragged into the LXE with the hope of making there wish to live forever together come true. The Hayasaka twins are bent on killing them, but due to Kazuki's kindness and not being able to kill Humans stop fighting. They give themselves into the Alchemy Regiment and in the end help Kazuki and his team find the head quarters of LXE. Kazuki, Tokiko and Bravo enter the LXE headquarters but they encounter only one Homunculus, known as Moonface. Bravo decides to stay behind and fight them while the others head back to the shcool to fend of the Homunculus attacking there(Trying to give power to Victor so he can be restored). Chouno however wants to figure out the truth of becoming the ultimate being, such as victor. So he fights Dr. Butterfly, and kills him. However his wish is denied.
Meanwhile Kazuki and Tokiko successfully fight off the incoming invasion to the school, but are to late as Victor is revived. He has the ability to drain energy of those around him(It's like breathing for him, he can't control it). Kazuki wants to save everyone so charges in but is beaten easily by Victor. Kazuki not wanting to give up figures out the true power of his Kakugane. It turns out to be one of the three Black Kakugane and he transforms into the same being as Victor.
Kazuki is now determined to figure out the truth of his "Victorazation" and the Black Kakugane. With Tokiko by his side the two set off on a journey of perilous danger.