Every time someone falls in love, a red line— like a tally mark, appears on their wrist. 

And if that love is requited, the line turns black. 

And if one of the lovers dies, the tally mark on the surviving one turns into a scar... 

Luhan has a single scar etched into the skin of his left wrist, and since the day he got it he refused to ever fall in love again. 

Until one day a black tally mark suddenly appears right below the scar.