Kazuki gets killed in episode one and comes back wrong, his entire arm replaced with alchemy that wasn't supposed to work on humans. The show builds its action around that violation, turning every fight into a scramble where he's figuring out what his own body can do while something much bigger circles closer. It's predictable shonen framework, but the fight choreography is tight enough that you stop caring, and the tonal shifts between comedy and genuine dread actually land.

Kazuki Muto, a high school student, is killed one night while rescuing a mysterious girl from a monster. He wakes in his dorm room and assumes the whole thing was a dream, until a huge serpentine creature attacks him and his sister. The girl, Tokiko Tsumura, explains that the creature is a homunculus and that she revived him by setting a Kakugane, an alchemical medallion, into his chest to serve as his heart. Once activated, a Kakugane takes a form shaped by its user and becomes a weapon called a Buso Renkin, and only such a weapon can kill a homunculus. Kazuki's is the Sunlight Heart, an enormous lance, while Tokiko fights with a scythe known as the Valkyrie Skirt. Dragged into a battle he never expected, Kazuki leaves his easygoing life behind and stands with Tokiko against the homunculi and their master to protect the people he cares about. The series blends supernatural action with comedy.