In I, THE EXECUTIONER, Detective Seo Do-cheol (Hwang Jung-min) and his major crimes unit decide to recruit young officer Park Sun-woo (Jjung Hae-in) to track down a serial killer who has plunged the nation into turmoil. When the murder of a professor reveals connections to previous cases, suspicions around the serial killer become more intense as he publicly taunts the duo and their team by announcing his next victim.