Plot[]
The Day He Got a Little Sister[]
Mio's method of waking up Basara.
Having finally moved in with Basara's family, Mio decides to begin the first day as siblings by waking up the young man in the morning, doing so by straddling his lower body as a greeting. Calling it 'service on the first day of cohabitation', Mio's friendliness quickly turns to outrage as she pulls off the blanket covering Basara, only to find several erotic magazines underneath. Seeing that they all had to do with little step-sisters, Mio accuses him of wanting to attempt lewd acts with them and begins to throw the magazines at Basara. In the commotion, both teenagers lose their balance and fall out of the bed, further angering Mio when Basara comes out on top. Coming downstairs for breakfast, Mio then finds Basara being teased by Maria and wonders if he's 'making a move' on her next.
Mio and Basara riding together.
Before leaving for the day, Jin gets the group to take a photo together to commemorate their new family, promising to send the photo to the girls' mother overseas. Basara then takes Mio out on his bike, asking her to guide him around town while they do some shopping. Taking the opportunity to ask him about the dream he was having that morning, Basara vaguely describes it as probably being the result of a trauma he suffered as a child.
After completing their shopping, Basara momentarily leaves Mio to unchain their bike. While he's gone, several thugs take the chance to move in on Mio, trying to convince her to go with them. Returning with the bike, Basara asserts that she's with him, and unhesitatingly strikes one of the men with the front tyre of the bike, surprising Mio. Throwing an open container filled with pepper at them, Basara then grabs Mio and rides away safely. Apologizing for causing trouble, Basara assures Mio that it wasn't her fault.
Mio and Basara at sunset.
Despite his reassurance, Mio remains upset over what had just happened. Suddenly declaring that they're taking a detour, Mio is then taken to an open park with a clear view of the city at sunset. Explaining that he'd heard about this famous spot before moving in, Basara then promises to save Mio no matter what, as it's only natural for families to help each other out. Smiling, Mio tells Basara that he's actually sounding like an older brother now. Before heading back home, Mio takes one last sad look at the scenery.
Mio bidding Basara farewell.
Later that night, after Jin leaves town for a business trip, Maria and Mio agree to 'begin'. Basara then re-enters the house after seeing his father off, and Mio remains awfully quiet while Maria asks if Jin has finally left. Finally speaking up, Mio then requests for Basara to leave the house immediately. Before he can respond, Maria uses an invisible force to throw him against the wall, stunning the young man. Explaining that it was magic, Maria transforms into her succubus form before revealing to Basara that Mio is, in fact, the future Demon Lord. In shock, Basara asks Mio if it was all true. Without any emotion, Mio coldly apologizes before saying goodbye as Maria activates her memory spell.
Surprisingly, Basara is unaffected by Maria's memory magic and quickly goes on the offensive, revealing himself to be one of the 'heroes' who oppose demons. Summoning his weapon, the young man goes to attack Mio with Maria jumping in between them. Before landing his attack, Basara stops and tells them he doesn't care that they're demons but asks them to leave due to deceiving him and his father. Without further conflict, Mio and Maria leave the house.
Mio and Maria being attacked by demons.
Going to the park Basara had shown her earlier, Mio asks Maria that they not deceive anybody anymore. However, before they can move on Mio notices that a magical barrier has been erected, and from behind them comes a shadow demon, and two manticores. Commenting that she'll never forgive them for the feud over her parents, Mio promises to kill them a hundred times over as she becomes cloaked in a shroud of power.
Mio being caught in mid-air by Basara.
Fighting off the manticores and seemingly destroying them along with the shadow, Mio is surprised to see the shadow demon suddenly reform, realizing it had used barrier magic to survive her attacks. Landing a blow, the shadow demon paralyzes Mio and sending her flying over the nearby cliff. Wondering if this is how she will die, Mio is then caught out of the air by Basara who had just arrived. Killing the shadow demon with his summoned sword, Basara tells the two girls to forget about before and offers to take them home.
The Way to Conquer Your Stepsister[]
Mio being reassured that they are family.
Once returning home, Mio takes a bath while Maria and Basara talk in the kitchen. Finishing in the bath, Mio thinks over the past six months as she dries herself off, comparing her first real battle with all of the training she had gone through with Maria. Realizing that she would have died without Basara's intervention, Mio grips herself to try and stop the shaking. While dressing, Basara knocks on the door and speaks with her through the wall, apologizing for sending them out without thinking. Trying to refute his apologies, Basara deflects her arguments by commenting on how there were quicker, more violent means to remove him from the house, noting her kind personality. Reminding him that the remarrying of their parents was just an act and that they weren't family, Basara simply says that wanting to protect each other makes them family.
Mio participating in the contract ritual with Basara.
Maria then interrupts the moment, snickering about how well the two were getting along. Before Mio can retort, Maria continues on to explain how there may be the time when Basara and she are separated, and that while there isn't a magic to perfectly locate someone, they may be able to achieve something close. Revealing it to be contract magic specific for masters and retainers, Basara becomes uncomfortable about having their souls united and asks Mio if she is okay with it. Mio then responds that she doesn't mind it, surprising the young man. Proceeding to begin the ritual, Maria supports Mio as she begins to cast the spell. Things don't go as planned however, as the mark for the master appears on Basara's hand - sending the two demon girls into a panic. As the mark on his hand begins to disappear, Mio refuses to submit to the ritual despite Maria's persuasion.
Mio's retainer mark activating.
Disappearing entirely, Maria comments that this isn't good. As she says this, the mark for the retainer appears around Mio neck and seemingly activates. Concerned for her, Basara reaches over and touches her shoulder, causing Mio to suddenly cry out in seeming pleasure. Confused by this, both Mio and Basara then listen as Maria explains that it's the result of the 'curse', the effects of the contract should the retainer disobey the master, in this case, Mio refusing to submit to the ritual. Due to Maria being the one to cast the spell, the curse would then have the aphrodisiac effects of a succubus, drastically increasing Mio's sexual desire.
Panting, Mio begs them to help her. Telling Basara that she'll eventually die at this rate if they don't do something, Maria explains that in order to lift the curse he'll have to make her obey him by touching her body. Embarrassed, Mio demands to know what Maria is planning, noting the little succubus' apparent delight at the situation. Despite the curse affecting her state of mind, Mio tries to resist the plan until Basara finally makes contact, rubbing her arms. While Mio moans loudly, Maria quietly tips off Basara about Mio's sensitive breasts which once touched quickly sends her into a frenzy. Still refusing to obey Barasa, Maria becomes concerned that the curse won't be lifted and so suggests that he touch Mio's breasts directly, which results in Mio continually crying out 'big brother!'.
Mio taking out her anger on Maria.
Eventually, after bringing Mio to the edge nine times, the curse is finally lifted. Left to sleep on the couch due to exhaustion, Mio recovers soon enough and drags Maria away, chastising her over the entire ordeal.