It's their first summer break as siblings. Precious days pass as they slowly grow closer.It's Yuuta and Saki's first summer break since they became step-siblings. Saki suddenly shows up, submits her curriculum vitae, and begins working as Yuuta's junior at the bookstore. Yuuta begins to interact with her as a senior and not as an older brother, seeing new sides of her that he previously didn't know existed.Then one day, while working the same shift as Saki, Yomiuri Shiori notices something inauspicious in Saki's actions and brings it to Yuuta's attention."That girl is always serious, she's way too strict to herself, and this solitary personality will cause her problems one day."Now Yuuta is forced to make a decision. No expectations, no overbearing meddling---Should he break their initial promise and support her in a way that would change her way of living, or let her continue to her own ruin?What is the choice he makes as her older brother, and what is the result?