At its core, Kakkoii Onna no Ko to 12cm no Yakusoku is a romantic comedy-drama that hinges on a single, specific physical detail. The story centers on a high school boy named Itsuki, who is notably short for his age, and his childhood friend, the tall and strikingly cool girl named Shiori. Shiori is the embodiment of the "kakkoii" girl: athletic, effortlessly popular with other girls, and possessing a calm, confident demeanor that makes her stand out. Itsuki, in contrast, is more reserved and often feels overshadowed by her presence, despite their long history. The central conflict arises from a childhood promise they made: that they would get married when they grew up. The catch is that they set a condition based on their heights. Shiori, who was always taller, agreed to the promise under the playful stipulation that Itsuki would have to grow to be within 12 centimeters of her height. Now in high school, Shiori has stopped growing at a statuesque 172 centimeters, while Itsuki has remained perpetually stuck at 160 centimeters, leaving a gap of 12 centimeters exactly. This makes the promise simultaneously impossible and binding, as the 12-centimeter gap itself has become an unspoken symbol of the distance between them.

The setting is a typical Japanese high school, providing the backdrop for the evolving dynamics of their relationship and the introduction of a supporting cast that complicates their bond. The narrative follows Itsuki’s struggle with his feelings of inadequacy, not just about his height but about his perception of himself as someone who cannot stand on equal footing with Shiori. He is constantly confronted by the admiration she receives and the ease with which she navigates social situations, often leading him to misinterpret her protective actions as pity. A significant character is a new male student, a stylish and conventionally handsome transfer student who is also tall and immediately shows an interest in Shiori. His arrival forces Itsuki to confront his feelings directly, as he fears losing her to someone who seems to physically match her better. Conversely, Shiori, despite her cool exterior, harbors a deep-seated insecurity about her own height, feeling it makes her less feminine and unapproachable, and she is quietly terrified that the promise will become a permanent barrier.

The major narrative arcs are structured around the changes in their relationship. The first arc focuses on their awkward reconnection and the re-establishment of their daily interactions, highlighting the comedic and painful moments caused by their height difference. This is followed by an arc where the transfer student’s presence acts as a catalyst, forcing both Itsuki and Shiori to acknowledge their jealousy and unspoken attraction, leading to a series of misunderstandings and near-confessions. A subsequent arc delves into their past, revealing the specific circumstances of the promise and other childhood memories that give the number 12 cm more emotional weight, showing it represents not just a measurement but a promise of future togetherness that now feels fraught with anxiety. A pivotal arc concerns Shiori’s involvement in school sports, where her talent is both celebrated and a source of alienation, and Itsuki must find a way to support her without letting his own insecurities overshadow his actions. The most significant conflict arc involves Itsuki making a decision to actively try to grow taller through absurd and often humorous methods, only to realize that the real issue is not his physical height but his emotional maturity. This leads to the climax where he finally confronts Shiori, not to renegotiate the height condition, but to redefine what the promise means to him, focusing on his determination to be a person worthy of her respect and love, regardless of the centimeters.