The story follows the Nakano quintuplets during their final year of high school. As the new year begins, Ichika, Nino, Miku, Yotsuba, and Itsuki realize that their time together is quickly running out with graduation approaching and their lives about to take separate paths to college and new careers. Feeling the weight of this impending separation, the sisters decide to establish a monthly tradition called Quintuplets Day, a dedicated time each month for the five of them to gather, share meals, and simply enjoy being together.

The narrative unfolds across a full year, from January through December, with each episode or chapter offering a glimpse into their changing relationships during this transitional period. Different segments are told from the perspective of individual sisters, providing insight into their inner thoughts and feelings about growing up and moving on. The series explores the bittersweet reality of a bond that remains strong even as physical distance begins to play a larger role in their lives.

The main characters are the five Nakano sisters: Ichika, the eldest who often takes on a caretaker role; Nino, whose straightforward personality hides a deep sisterly love; Miku, the more reserved and introspective one; Yotsuba, the energetic sister who struggles with her past self; and Itsuki, the youngest who faces the pressure of university entrance exams while longing to maintain family connections. Their daily lives at home, study sessions, holidays, and seasonal events form the backdrop for stories about sibling bonds and personal growth.

Several notable narrative arcs shape the series. One early episode focuses on Ichika and Itsuki, revealing why Ichika affectionately calls her youngest sister Itsuki-chan and exploring the dynamics between the oldest and youngest siblings as they help a stressed Itsuki cope with exam pressure. A Valentine's Day arc centers on Itsuki and Nino, showing how the sisters support each other during the stressful days leading up to university entrance exams, with Nino revealing a more nurturing side and Itsuki learning to accept help and express her feelings honestly. A graduation arc follows Yotsuba as she confronts her feelings of loneliness about leaving high school behind and learns to make peace with her past mistakes and insecurities.

The setting is primarily the Nakano familys apartment and various locations around their city as the sisters navigate the changing seasons. From New Years shrine visits and spring cherry blossoms to summer festivals and winter graduations, the story captures the passage of time through familiar Japanese seasonal traditions. The overarching theme is one of gentle farewells the acknowledgment that while their time living together as high school students is ending, the bond of sisterhood remains unbroken, merely evolving into something new as they each step into adulthood.