Joo Jaekyung is an elite, undefeated Mixed Martial Arts (MMA) Heavyweight Champion widely feared under the moniker "The Tyrant Emperor in the Ring." Fighting out of Team Black, he dominates his division through a combination of massive physical stature, explosive power, and an aggressively ruthless combat style that leaves opponents completely broken. He maintains an obsessive devotion to his training regimen and treats his athletic career with absolute seriousness, viewing victory as his only option. However, his flawless professional record is entirely bound to a psychological chokehold: a strict, secret "jinx" or superstition requiring him to satisfy specific physical needs the night before a match, without which he believes he cannot win.

Kim Dan is the 29-year-old physical therapist of the undefeated MMA champion, Joo Jaekyung, characterized by his slim build, fluffy brown hair, and deeply selfless yet submissive personality. Traumatized by lifelong poverty and abandoned by his parents, he accumulated massive, dangerous debts from predatory loan sharks to pay for his terminally ill grandmother’s medical care. After being blacklisted from the medical community for rejecting a corrupt hospital director's sexual advances, a desperate Dan is hired by elite MMA fighter Joo Jaekyung for an incredibly high-paying, coercive arrangement to help break Jaekyung's pre-match "jinx." Throughout the series, Dan struggles with immense emotional trauma and an intense fear of abandonment, a vulnerability that culminates in a heartbreaking separation after a mishap during a match sends him fleeing to a remote village.