Aika, the new younger sister of Kota Takamura from his father's remarriage, is a super sassy gal who bluntly calls her stepbrother “lame” upon first meet. Calling each other names, with each holding the worst impression of the other, they begin living under the same roof. But Aika would bring men into her room and get down to business whenever the parents were out. Kota is greatly troubled by her loud and clear moans, but then one day, something changes...

That day, Aika was indulging in intimacy with her boyfriend Takahashi in her room as usual. However, a certain situation stirs trouble between them. To make matters worse for Aika, Kota, who had heard the commotion, shows up at the door. Aika tries to brush him off and send him away, but Kota makes a bold move that takes them both by surprise.

Despite Aika skipping school and bringing boys home—behavior that could be called delinquent—she hides it from her parents and acts like a model daughter... Living together, Kota witnesses her dual nature firsthand. One day, Kota accidentally runs into Aika in the bathroom right after she's had a shower. Wondering which is the real Aika, he plucks up the courage to ask her directly...

That night after the incident in the bathroom, Aika and Kota each lay in their own rooms, lost in thoughts of the other. Aika replays Kota's words and actions in her mind, sensing something different from the guys she'd dated before—guys who were only after her body. Late at night, Kota goes to the kitchen to prepare instant noodles for a late-night snack. Then, Aika comes over...

Aika still lets Takahashi into her room, although today she seems unusually reluctant. The tactless Takahashi doesn't seem to care however, teasing that all guys get horny around the sexy Aika. Aika snaps and slaps him across the face. Takahashi then changes his tune, trying to force himself on her...

Kota narrowly defeats Takahashi and succeeds in protecting Aika. But now, with Aika still in her disheveled state, she asks him to hold her tight, leaving him flustered. Out of a sense of responsibility as her older brother, Kota tells her to put her clothes back on properly. However, after she had tidied herself, Aika demands he embrace her once more. Kota found himself cornered, with nowhere to escape.

The emotional and sexual tension reaches its peak: Aika and Kouta share tender and explicit moments, almost crossing the line completely, but with interruptions and hesitations. Their bond strengthens amidst the forbidden and the inevitable, with more romance beneath the fanservice.

The Takamura family heads to Atami for a family trip. Encouraging their parents to enjoy some time alone as a couple, Kota spends the day sightseeing by himself and fully enjoys the vacation. Later, as he returns to the inn to rest, Aika comes back to their room after taking a bath, dressed in a yukata. With a distinctly different atmosphere from her usual self, Aika’s traditional Japanese look catches Kota off guard, and he cannot help but be captivated by her appearance. However, Aika quickly notices his gaze…

The Takamura family continues in Atami! Aika and Kota arrive at the beach. While Aika is strolling alone in her swimsuit, she is approached by a man. Although she is annoyed, she tries to avoid the situation peacefully by saying, "I have a friend here," and leaves. However, the man, who is not very understanding, grabs her hand forcefully, and she finds herself in an unexpected pinch.

Now it's become perfectly normal for Aika to read manga in Kouta's room. She brings all the volumes out to read them all at once in the living room, but the moment she sees the dark hallway, she becomes strange and dejected, returning to Kouta's room. Aika confesses to Kouta that she's scared of being alone. Kouta assumes she just hates being on her own, but it seems there was more to it than that...

Aika realizes she feels both excited and satisfied from her physical closeness with Kouta. Then at that moment, the image of a certain guy who once boasted that he'll teach her what real love is, flashes through her mind... Kouta, having finished his part-time job, was about to head home when he coincidentally runs into Aika, who was visiting as a customer. They end up going home together, but Aika leads him to a certain place.

Aika boldly barges in on Kouta's bath time, holding a trendy bath bomb. Meanwhile, Kouta, concerned by Aika's tears he saw in the ruins, blurts out something that spoils the mood. However, Aika's reaction was different from what Kouta had expected. Sensing something, Kouta asks Aika to tell him about herself. Then, Aika begins to recount a certain event from her past.