One night, Kazuki Muto jumps in to try to save what he thinks is a girl in distress from a monster and instead gets impaled through the heart. The girl, Tokiko, is actually an alchemic warrior with a Busou Renkin, name of Valkerie Skirt, with the mission to exterminate Homunculi. She implants the only other successful work of alchemy than the Homunculus, the Kakugane, to take the place of Kazuki's heart. The Kakugane resonates with his fighting spirit and forms a Busou Renkin (in his case a spear) so he can fight the Homunculus monsters who prey on humans. Kazuki joins with Tokiko to battle with the Homunculi and protect his friends and his sister Mahiro.
This show has some decent characters with a few funny lines here or there but other than that it makes no logical sense. It starts off going down a interesting plot line then all of the sudden it takes a twist that makes at least half of the show completely pointless and leaves you with a feeling of feeling like wait what was the point of this or that and all you get is more questions than answers.