"My feelings... you knew how I felt, and yet you did something like this..." Mizuki stammered, her heart concealing a deep, hidden affection. Ever since childhood, walking hand-in-hand with her senior to school, she had gradually developed romantic feelings for her. Though she once believed they were merely friends, whenever she met her senior's gentle, warm gaze, her heart began to tremble. Attending the same school under a continuous education system, the two remained together even after graduation, building an increasingly intimate bond in their daily lives. One day after school, Mizuki was alone in the classroom, lost in thought, when her senior suddenly appeared and kissed her without warning. "Wha—what are you doing!?" Mizuki gasped in shock, her face flushed. Her senior simply smiled mischievously. "I've always thought of you as more than just a little sister, Mizuki..." From that day on, a secret, special relationship began—known to no one but the two of them. Her senior's pure, tender, and sometimes playfully affectionate advances slowly stirred Mizuki's heart. This is the story of their forbidden romance.