Da-in never expected to reunite with her first love, Seojun, under such bizarre circumstances. Claiming that marriage is the only way to escape his supposed fated death, Seojun proposes a 100-day contract marriage. Refusing to be at a disadvantage, Da-in negotiates her own terms and plunges into an unconventional newlywed life. What starts as a calculated agreement slowly unravels into something deeper, as unresolved feelings resurface and the countdown to day 100 threatens to change everything.