Todos os episódios

Aika, a nova meia-irmã mais nova de Kota Takamura, fruto do novo casamento do seu pai, é uma rapariga extremamente atrevida que trata o seu meio-irmão por "fracote" logo no primeiro encontro. Trocam insultos constantemente, cada um com a pior impressão possível do outro, e vivendo debaixo do mesmo teto. Mas Aika costumava levar homens para o quarto e ter relações sexuais com eles sempre que os pais estão fora. Kota ficava bastante incomodado com os seus gemidos altos e claros, mas um dia, algo muda.

Naquele dia, Aika estava num momento íntimo com o seu namorado Takahashi no seu quarto, como de costume. No entanto, uma determinada situação causa problemas entre eles. Para piorar a situação para Aika, Kota ouve a confusão e aparece à porta. Aika tenta livrar-se dele e mandá-lo embora, mas Kota faz uma jogada ousada que apanha os dois de surpresa.

Apesar de Aika faltar às aulas e trazer rapazes para casa — um comportamento que poderia ser considerado delinquente —, esconde isso dos pais e age como uma filha exemplar. Ao viverem juntos, Kota testemunha a sua natureza dupla em primeira mão. Um dia, Kota encontra Aika por acaso na casa de banho logo depois de ela tomar banho. Curioso para saber qual é a verdadeira Aika, ganha coragem e pergunta-lhe diretamente.

Nessa noite, após o incidente na casa de banho, Aika e Kota estavam deitados nos seus próprios quartos, perdidos em pensamentos um sobre o outro. Aika revia as palavras e ações de Kota na sua mente, sentindo algo diferente dos rapazes com quem tinha namorado antes — rapazes que apenas queriam o seu corpo. Pela noite dentro, Kota vai à cozinha preparar massa instantânea para um lanche noturno. De seguida, Aika aparece.

Aika ainda deixa Takahashi entrar no seu quarto, embora hoje pareça excecionalmente relutante. O indelicado Takahashi, no entanto, parece não se importar, provocando-o ao dizer que todos os rapazes ficam excitados perto da sexy Aika. Aika irrita-se e dá-lhe uma chapada na cara. Takahashi muda então de ideias, tentando forçá-la a fazer sexo.

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.