Shin Yuna's older sister, Shin Kyungsoon, was her hero. She saved Yuna, giving her attention every day and making sure her needs aren't unmet. However, although the two are sisters and inseparable, Kyungsoon has an intense secret that no one will ever know alive. Her job was dangerous, to say the least, but she had been doing it for over seven years. One day, her manager calls her in to perform a mission but it doesn't go the way she expected..