Set in Gyeongseong during the Japanese colonial period, the drama follows Ga Gyeong, the city's most talented pickpocket, who strikes a dangerous deal with the independence army and goes undercover at the Japanese Government-General of Joseon. Disguised as an employee, she is tasked with acting as a "fake spy" for just 100 days, but her mission slowly turns her into a real informant caught between survival, loyalty, and conviction.