Ever had a nightmare that felt so real you could have sworn that it wasn’t just a dream? Mutou Kazuki was in that predictament–however! It was not a dream.

As Kazuki begins to learn more about his enemy and the source of his power he decides to assist Tokito in the eradication of the Homonculi in his town. As Tokito begins investigating the “haunted factory” of the Homonculi she discovers a very disturbing thing, apparently the Homonculi have been developing a new weapon, a parasite that turns people into man eating monsters.

Muto Kazuki and Tsumura Tokiko are investigating who is behind all the homunculi appearances and hostile acts. Only with two leads: He wears a Papillion Mask and attends Kazuki’s high school. With that in mind, Kazuki and Tokiko do a thorough search of the high school–searching for more leads as to who this mysterious ” Papillion Mask Creator ” is.

After searching hard for the Papillion Mask, they finally found him–although he was definitely not what they were expecting. He supposedly wanted to bargain with Kazuki: the antidote to the Homunculus attached to Tokiko’s body for his Busou Renkin, but Kazuki refused. With a single Kazuki punch, The Papillon Mask was knocked out and brought to be questioned. (The “antidote” was a fake.

Kazuki and Tokiko retreat to form a plan on how to defeat the eagle Homunculus but not all goes as planned when Tokiko receives possibly critical injuries. Hurt but not out for the count the eagle Homunculus plans to target Tokiko, but Kazuki won't hear of it and they face off in an exciting and explosive duel. Kazuki learns the reason behind this Homunculus's awesome strength is quite similar to the source of his own will to fight. When the battle turns against him and all seems lost Kazuki decides to go for broke and unleash a powerful but dangerous attack.

Kazuki finds Koushaku in his great-great grandfather's laboratory and attempts to destroy the Homunculus matter. However, due to the interference of Koushaku's younger brother, Koushaku is able to have the Homunculus matter attach to him and is turned into a Homunculus. He then begins to eat everyone in the vicinity until he is killed by Kazuki, who passes out from exhaustion before he is able to deliver the cure to Tokiko. Kazuki awakens to find that Tokiko has been cured and that his buso renkin shall be called Sunlight Heart.