Miku and Kanna have been best friends since elementary school. Miku is responsible but has a habit of fantasizing, while Kanna is airheaded by nature but turns surprisingly naughty when serious—a mysterious girl. After enrolling in the same high school, Kanna easily befriends new classmates, sparking jealousy in Miku. Amidst this tension, they ask each other, "We'll always be friends, right? We're special to each other, right? Shouldn't we create a special sign just for us?" The answer they come up with is 'a kiss.' Their friendship gradually transforms into love, escalating into an intense, erotic relationship as their bond deepens in a new, passionate way.