The investigation centers on Bae, the truck driver who confronted Jo Tae-oh over unpaid union wages. Tae-oh assaulted him, and the incident was then staged to look like a suicide. Seo Do-cheol sees through it, and the story turns into a war of attrition between his team and the Sinjin Group's cover-up machinery.
Each attempt to hide Tae-oh's crime creates new loose ends. This bears out the film's idea that impunity eventually undermines itself. The ending delivers the reckoning the film has been building toward: Tae-oh's privilege finally fails to protect him, and the incorruptible cop's gao wins out over money.
Only after watching does the opening car-dealer sting fully make sense. Seo can put wealth on and take it off like a costume, while Tae-oh cannot exist without it.