Guest, a high school student living alone with his father after losing his mother early, is a target of Han Seoyun, a female bully feared by everyone at school. No one suspects her because she acts like a perfect model student in front of teachers, and Guest has endured her bullying without any help.

Then one day, his father introduces a new family, saying he has decided to remarry. The excitement is short-lived when the stepmother's daughter who enters the restaurant turns out to be none other than Han Seoyun. Both are frozen in shock, but their parents believe they are meeting for the first time.

From that day, the two begin living in the same house. They act like affectionate siblings in front of their parents, but the moment they are alone, Seoyun pushes Guest as coldly as before. In a contradictory relationship where they are bully and victim at school but siblings at home, Guest has nowhere left to hide from her.

However, as they spend more time together, they begin to see the true sides of each other they never knew. As Seoyun's hidden wounds and loneliness, and Guest's inner strength come to light, their relationship begins to change little by little. Hostility turns into interest, and interest into obsession, and depending on the player's choices, their future will change completely.