Suddenly red lines appear above people's head. These red lines connect people who have had sex relations. Due to this, embarrassing or secret sides of people are revealed. People call these red lines "S Line."

Han Ji-Wook (Lee Soo-Hyuk) is a detective with an attractive appearance and free spirit. He chases after the truth of the S Line. Kyu-Jin (Lee Da-Hee) is a high school teacher with unique charm. Hyun-Hob (Arin) is a high school student. She has been able to see the red lines since she was born.