Stranger follows Hwang Si-mok, a prosecutor who underwent brain surgery as a child that left him devoid of emotions and social empathy. Despite his cold exterior, he is highly rational and possesses an unparalleled ability to analyze crime scenes. When he encounters a murder case that leads him to a larger conspiracy within the prosecutor's office, he teams up with Han Yeo-jin, a tenacious and warm-hearted police lieutenant. Together, the duo navigates a web of corruption, political maneuvering, and systemic injustice. As they peel back the layers of the mystery, they must rely on each other's contrasting strengths to expose the truth, all while facing immense pressure from powerful figures determined to keep their secrets buried.