The story begins in the spring of a second-year high school student, Hanano Akari's, life. Akari is utterly captivated by the beauty of the Tsukimori triplet sisters, who are renowned as the most beautiful girls in school. Her joy is amplified when she discovers she's been placed in the same class as them. Akari's intense admiration for the sisters is soon noticed by a "yang-ki" (energetic and popular) girl, who confronts Akari, finding her devotion to the Tsukimori sisters abnormal and uncomfortable. Akari tries to explain that her feelings are not romantic but a pure, all-encompassing admiration, akin to a mother's love, but the other girl misunderstands and, to Akari's horror, drags the Tsukimori sisters over to confront her.
The eldest sister, Chiyo, coldly dismisses Akari's feelings as unrequited love, while the second sister, Hiyori, offers a polite but distant apology. The youngest, Karin, simply tells Akari "Don't worry." The yang-ki girl mocks Akari's rejection, leaving her devastated.
Returning home, Akari is shocked when her mother announces she's remarrying. The situation escalates rapidly as her mother insists on an immediate meeting with the new family. They arrive at a large house, and Akari is stunned to discover the groom is the father of the Tsukimori sisters. She has now become their stepsister, making them the "Tsukimori Four Sisters."
Adjusting to her new life, Akari is shown to her room by Chiyo, who makes it clear she doesn't intend to accept Akari as a sister. Akari, however, readily agrees, stating she's content to be a mere cohabitant and doesn't harbor any improper feelings, which surprises Chiyo. Later, Hiyori kindly shows Akari the bathroom, though her gentle words subtly imply Akari's lack of basic awareness. Karin assigns Akari household chores, including cleaning the bathroom, and surprisingly refers to her as "little sister," which pleases Akari despite Karin's gruff demeanor. Akari's initial goal becomes dispelling the misunderstanding that she has romantic feelings for the sisters.
The next day at school, rumors of Akari's confession spread, leading to her ostracization. The Tsukimori sisters, meanwhile, maintain their distance from her. Akari observes that the sisters, despite being triplets, don't always act in unison, and Karin hints that their relationship was closer in the past. Akari becomes determined to investigate this potential rift.
During a physical education class, Karin shows concern for Akari's poor physical condition and offers to listen to her troubles, though Akari deflects. Later, Akari witnesses Karin struggling in basketball practice, unable to overcome defensive players. Akari, despite her own lack of athletic ability, challenges Karin, claiming she can stop her. This leads to a playful but intense confrontation where Akari manages to momentarily impede Karin's progress. Akari uses this opportunity to advise Karin to broaden her perspective, both in basketball and in her relationships with her sisters, suggesting that her rigid focus is hindering her. Karin, though initially resistant, seems to consider Akari's words and acknowledges her defeat, even calling Akari by her name for the first time.
Akari continues to try and bridge the gap between the sisters. She observes that while Karin has become more open, Chiyo remains distant, and Hiyori's gentle demeanor sometimes hides sharp remarks. During dinner, conversations are sparse, but Karin initiates dialogue, and Hiyori prepares lunchboxes for everyone, including Akari. Akari's efforts to foster better relationships among the sisters are ongoing. She notices that Karin's newfound openness is starting to affect Chiyo's reactions, and Hiyori's actions, like making Akari's bento, show a growing acceptance. Akari's ultimate goal is to help the Tsukimori sisters reconcile their strained relationships.
