A high-stakes legal battle unfolds as a lawyer must save a genius serial killer to protect his daughter, while an elite prosecutor fights to sentence him to death.

Su-ho and Hyeon-seo, who once dated for six years, meet again as a youthful rich CEO and a piano instructor. K works with Su-ho on a collaborative project and invites Hyeon-seo to join them. Su-ho and Hyeon-seo embark on a bumpy romantic journey full of twists and turns. Will history repeat itself?

In 1933, when Korea was under Japanese occupation, five people in Gyeongseong are suspected to be "Phantom" spies of the anti-Japanese organization.