The show will tell the story of a mysterious red line that appears above people’s heads, connecting them to those they’ve had sexual relations with. Han Ji Wook is a detective who wants to find out the truth about the mysterious red line. Kyu Jin is a quirky high school teacher and Hyun Hob is a high school student who has been able to see the red line since birth. Adapted from the webtoon "S Line" (S라인) by Little Bee (꼬마비).