Momo once said the ghost of us haunted her. The memories of a love with Sana that felt eternal but suddenly shattered. She and Sana had been inseparable from high school all the way through college. Sana was her "Pres" her person, the one she thought she'd grow old with. But everything changed. Out of love and fear, Sana pushed Momo away, pretending she'd fallen for someone else, hoping to spare Momo the pain of watching her fade.

Detective Yoo Jeongyeon is hot headed, firmly believes in instant justice and that no crime is forgivable regardless of its size. She gets transferred from Seoul to the Jung District in Busan after shooting a rapist to death. Son Chaeyoung, the youngest detective in the homicide squad, is her polar opposite, she believes in kindness and compassion, she believes in second chances. They get partnered together to investigate the serial killings of young men and women in the city which is nicknamed the Cupid Murder because all the victims had been stabbed in the heart. What happens when everything points to Im Nayeon, the powerful, right hand of a drug lord? Will Jeongyeon and Chaeyoung overcome their differences to see justice served?