Less Than Evil is a gritty South Korean crime thriller that follows Woo Tae-seok, a cynical and obsessive detective who is willing to cross moral and legal lines to catch criminals. Haunted by a cold case from his past involving a young girl, his life takes a dark turn when he encounters Eun Sun-jae, a brilliant but psychopathic journalist who he suspects is a serial killer. As the series progresses, Tae-seok finds himself in an uneasy, cat-and-mouse alliance with Sun-jae. While he attempts to use her intellect to solve complex cases, he must grapple with the realization that she may be the very person who holds the key to his past trauma. The drama explores the thin line between justice and vengeance, questioning whether one must become a monster to defeat one.