She is Mr. A, the ace agent of Section 21, and he is the captain of the Criminal Investigation Team. They are destined to meet again two years later. She is still the agent who is the best at discerning people's hearts, and he, who used to be unruly and unruly, is now vicious and vicious... She wants to clear the wrong of her best friend K, and he wants to find out the cause of his parents' death. Little by little, he gradually discovers that she has been in the same bureau since a long time ago...