In another universe where soulmates exist and are identified by marks in the form of whatever injury your partner has- Chuuya Nakahara is about to turn 16, and his best friend Dazai has been endlessly teasing him over the prospect of a mark. However, when he wakes up on his birthday covered in light blue lines he is unsure what to do, or how to approach identifying his soulmate, and his best friend has been acting in increasingly concerning ways since being told about the marks.