Cheon Suji, a young professional just starting out, struggles through each exhausting day—burdened by endless overtime and unwelcome weekend hikes. One day, her older brother's friend, Seo Jaehyun, reappears in her life after a long absence. Handsome and affectionate, Jaehyun had always doted on Suji, who is ten years his junior, ever since she was a child. Was it the alcohol? On impulse, Suji kisses the sleeping Jaehyun—only to get caught. Expecting him to recoil in disgust or even accuse her of harassment, she braces herself. But instead, he says something completely unexpected: "Who kisses without even opening their mouth?" "Then... how are you supposed to do it?" "Open your mouth." In that moment, as Suji is swept away by desire, Jaehyun begins saying things she never could've imagined. Her first love of 25 years—a total pervert?! Yet rather than being repulsed, Suji finds herself awakening to hidden desires of her own. Though she tells herself she must push him away, she becomes increasingly entangled in his relentless, seductive pursuit…