An epidemic of unknown origin strikes mankind, killing all adults and rendering the youngsters at the mercy of the vampire kind. Four years later, Yūichirō Hyakuya and his friends make a daring attempt to escape from the vampires, unaware that a mortal trap awaits them.

Four years after escaping, Yūichirō is part of the Japanese Army and is eager to become a member of their special vampire hunting force, the Moon Demon Company, but his reckless behavior leads him to be suspended, until a vampire attacks the local school prompting him to face the enemy against orders.

Yūichirō learns that one of his schoolmates was possessed by a demon and decides to confront him not only in order to save him but to make a contract with the demon and obtain his power. However, he soon learns that subduing the demon will not be such an easy feat.

Instead of dying with the rest of his friends, Yūichirō's best friend Mikaela was transformed into a vampire by Krul Tepes, the queen of the vampires and now works as her aide. Meanwhile, Yūichirō finally begins his training to become a member of the Moon Demon Company.

Wary of the Human Army's rise to power, Krul calls the vampires to war against them, while Yūichirō and his friends are finally selected to attempt to claim a demon weapon for themselves, despite knowing the risks.

After passing the final test, Yūichirō and his classmate Shihō Kimizuki obtain their own Demon Weapons, but when Yūichirō's friend Yoichi Saotome ends up possessed by his weapon, the duo must find a way to bring him back to his senses without killing him.

Yūichirō and his friends introduced to a new member, Mitsuba Sangū, and are then dispatched for his first mission as part of the Moon Demon Company, but when they spot a little girl in distress, Yūichirō ignores orders and rushes to rescue her, despite being warned that he is just being lured into a trap.

After successfully clearing a vampire nest and rescuing the humans held captive, Yūichirō's party sets for the human base in Shinjuku, unaware that the vampires are attacking there. Upon arriving, they come across one of the vampire nobles, and find themselves at mercy of his overwhelming power.

Yūichirō and Shinoa get separated from the others and after taking a wounded soldier to be treated, they regroup to assist Guren's party. In the occasion, Yūichirō and Mikaela finally meet again.

Mikaela attempts to escape, taking Yūichirō with him. However, seeing his friends at the mercy of the vampires, Yūichirō goes out of control when a dreadful power emerges from him, threatening allies and foes alike. Shinoa manages to stop him but he falls unconscious.

Yu has been in a coma for five days when Shinoa confronts Guren about special drugs in Yu's pill. He tells her to go and wait for Yu to wake up if she really cares for him. During the seventh day of his coma, Yu wakes up. He finds out that Mika retreated with the other vampires. Once all his friends have gathered he thanks them and apologizes to them. At the end of the episode both Yu and Mika promise themselves they will save the other.