After returning from his last job, Mardock, A Demon-Slayer who became famous in Nardalach after slaying an army of one hundred demons all by himself, gets attacked by a beautiful and young girl called Sapphire, a demon from the strongest race of the demon world, the Daemon race – a race of demons that greatly resemble to humans. Sapphire wants to kill Mardock for glory and to be known as the most powerful demon in the demon world, however due to her arrogance, Mardock succeeds to paralyze her and put her at his mercy, but Instead of killing her, Mardock pities her and decides to seal her powers with a special seal. The seal does not only take away Sapphire's powers, but also turns her into a child and gives her emotions.
Now in the state of a powerless child, Sapphire can't return to the demon world and since she wants Mardock to release her from the seal, she follows him to his house and he promises her that if she learns to behave, he might unseal her powers. At the Demon-Slayer's house, Sapphire meets his two young apprentices: The kindhearted Rye, who has magical powers and Sapphire befriends with him rather quickly; and Iris, who at first bullies Sapphire and resents her greatly because of her being a demon, but later, Iris realizes that her behavior was no different from that of a demon and she becomes nicer towards Sapphire, and slowly, the two become good friends.
Sapphire is really upset and depressed about losing her powers and being turned into a little girl. Much to her annoyance, the Demon-Slayers also treat her like a child, though without her notice, she does act rather childishly.
A mysterious masked man, named Xor, appears and he's interested in Sapphire. He demands to have her, and after Rye immediately refuses, the two engage into battle. Rye barely succeeds to hold his own against his opponent, and is saved when Iris and Mardock come to his aid. Xor flees from the battle, knowing he can't defeat the three Demon-Slayers together.
As time passes, Sapphire gets used to being a child and starts to enjoy her new life in the human world, and when her servant from the demon world, Laila, comes to take her back, Sapphire says that she wants to stay in the human world for a little longer.
The joy however ends when Xor returns with a group of half-demons, hybrids of humans and demons, and attack the Demon-Slayers. Rye especially has difficulty fighting the half-demons because he doesn't want to kill those with human blood due to his belief that whoever is human, or even partly human, can be a good person. The Demon-Slayers succeed to defeat the half-demons, but Xor manages to wound Mardock fatally, and the elder Demon-Slayer dies without releasing Sapphire from the seal.
With Mardock's death, Sapphire becomes depressed again, because now she has no way to become an adult, and since demons can't grow up in the human world, if she ever wants to grow up, she has to go back to the demon world, but she became so close to Rye and Iris and doesn't want to leave them. Then she realizes that she endangers the Demon-Slayers by staying with them, and decides, for Rye's and Iris's sake, to go back home. While Iris and Rye mourn about their dead master, their relationship develops, and the two finally find the courage to confess to each other about their feelings for each other.
While Sapphire waits for Laila to come to pick her up, and bids farewell to the Demon-Slayers, Xor's minions attack again. Without Mardock, Rye and Iris can barely handle their enemies. The hybrids succeed to overwhelm the Demon-Slayers and kidnap Sapphire, and Rye blames himself for what happened.
The Demon-Slayers immediately set to rescue Sapphire, but just as they are about to leave, a horde of half-demons attack the village, and the Demon-Slayers are forced to stay and defend it from the hybrids. The true intention behind the attack is to delay the Demon-Slayers from getting to Sapphire so Xor's plan can be fulfilled. At his hideout, Xor offers Sapphire to give her back her powers in exchange for helping him in his goals, but Sapphire declines his offer, however after Xor feeds the naïve Daemon with lies and illusions, and makes her think that Rye and Iris don't really care for her, Sapphire agrees to Xor's deal. Unknown to her, Xor also intends to control her and use her in order to destroy the human world.
After Reinforcements arrive to help at the village, Rye and Iris go to Xor's whereabouts and fight through his remaining pawns, but just when they arrive, Xor succeeds, via magic, to return Sapphire back into an adult along with her powerful demonic abilities, however to his dismay, he can't dominate her, and Sapphire attacks Xor and defeats him easily. Then Sapphire, still influenced by Xor's lies, attacks Rye and Iris with hatred, but the two don't fight back and try to talk to her and make her realize that they care about her. At first, Sapphire doesn't listen, but after awhile, she realizes that she doesn't hate Rye and Iris, and she doesn't want to kill them. Sapphire apologizes and the Demon-Slayers immediately forgive her. Even though she's now an adult again, she feels unhappy, and realizes that when she was a child, she was happy, so she decides to turn back into a child and live with Rye and Iris who care for her, knowing that she will find much more joy in the human world than in the demon world.
