Diary of a Prosecutor tells the story of overworked prosecutors’ daily lives, who are just like you and me, ordinary folks in the city of Jinyoung.

Lee Sun-woong (Lee Sun-kyun) is a passionate prosecutor at the district public prosecutor office in Jinyoung. He comes from a wealth background and was acquainted with Cha Myung-joo (Jung Ryeo-won) in university. However, they are always on loggerheads.

Cha Myung-joo is a very successful prosecutor in Seoul. However, due to an incident, she was requested to join the district prosecutor office in Jinyoung. She does not like to waste useless time on cases. In some of her colleagues’ eyes, she appears to be careless prosecutor.

Jo Min-ho (Lee Sung-jae) is the chief of the district prosecutor office in Jinyoung and has been a prosecutor for 18 years. He hates seeing his team coming second in the office. After Myung-joo’s incident in Seoul, he convinced her to join them at Jinyoung with the intention that her arrival will raise his team’s results and be number one.

Hong Jong-hak (Kim Kwang-kyu) is a prosecutor with 14 years of experience.

Oh Yoon-jin (Lee Sang-hee) is a mother who has been a prosecutor for 4 years.

Kim Jung-woo (Jeon Sung-woo) is the youngest prosecutor at the district public prosecutor office in Jinyoung. Myung-soo becomes his mentor, after she joined their office, taking over from Sun-kyun. He enjoys dating but his workload stops him from doing so most of the time.

Will Lee Sun-woong and Cha Myung-joo ever make up and be able to work together?

Can Jinyoung District Public Prosecutor Office be able to get the first spot with Myung-joo joining the team?