Transitioning from childhood to adolescence, twelve-year-olds Hanabi Ayase and Aoi Yui encounter pivotal moments that shape their lives. Ayase's distrust of boys due to her classmates' antics wanes when she partners with the kind-hearted Takao Yuuto for a project, leading to an unexpected shared kiss on the school rooftop. Meanwhile, Aoi witnesses her best friend's budding romance and finds herself drawn to the impulsive Kazuma Hiyama after he defends her from a gossiping suitor. In '12-sai.: Chicchana Mune no Tokimeki,' the quartet navigates the complexities of early teenage years, exploring the tender emotions and thrill of first love as they mature together.