Mahiru is a boy who loves simple things and hates troublesome things. He decides to take a black kitty home, but soon finds out that his new pet Kuro may be one of the most troublesome things of all.
Mahiru's pet Kuro turns out to be a servant vampire, or servamp. Mahiru rushes back to school to find out what happened, but his friends do not remember. All Mahiru knows is that a person named Tsubaki is after Kuro...
Mahiru finds out that his best friend, Sakuya, is actually one of Tsubaki's subclass. Why did he get close to Mahiru in the first place? Is Sakuya truly Mahiru's friend or is that another one of the lies that Sakuya hates so much?
Mahiru nearly gets consumed by the power that he had asked for, but a stranger with the servamp of Envy comes to save them in the nick of time. Mahiru realizes that just having power doesn't solve anything and struggles to figure out what to do next.
Mahiru continues to meet new people one after another. Next, he meets a strange boy carrying a coffin at the summer festival as he tries to turn in a lost suitcase given to him by a stranger. People start to take action, each with their own reasons.
Mahiru meets with another suspicious character, a visiting pianist from Austria. He comes to find that the pianist, Licht, is actually the Eve of Greed, Lawless. However, Lawless isn't as friendly as the other siblings Mahiru has met so far.
Kuro continues to act strangely ever since Lawless brought up the past. Mahiru steps gingerly around Kuro, unsure whether to ask about his past or not. All the while, Tsubaki moves to take revenge on Lawless and his Eve, Licht.
Kuro has been trapped away in a ball and the others will have to try and rescue Licht and Lawless before midnight without them. As they plan, Mahiru and Kuro head to the shop of a certain antique dealer, to find a way to get Kuro free.
Misono and Tetsu go to save Lawless and Licht while Mahiru wanders within Kuro's mind to find a way to save him. Misono realizes how little they know of Tsubaki's intentions and Mahiru realizes just how much resolve it takes to face Kuro head-on.
Licht breaks through the wall to give Lawless a piece of his mind, face-to-face. Licht gets swallowed up by the darkness and the curtain rises on Lawless' past. What is it be unique? How can one become the one and only?
Licht and Lawless fight against Higan while Hugh lets his guard down due to the prideful idea that subclass could never be a match against him. Misono panics. Mahiru and Kuro arrive on the scene to help, but perhaps a bit too late.
The final battle is here and Mahiru struggles to get to Kuro as he steps forward and tries to face it all. Kuro faces his past, his neglected little brother Tsubaki, and his own actions which may have led to who Tsubaki is now.
