Slow Damage takes place at some point during the 21st century in the fictional ward of Shinkomi, which is located in Tokyo, Japan and was previously a part of the Tokyo Bay City No. 13 subregion. The country is in the midst of a recession and, in an attempt to resolve these economic troubles, the government repurposed this area as a large-scale casino resort due to its historic prosperity. This ward is governed by the corrupt Takasato Group whose only interest is their own financial gain.

The story follows Towa, a resident of Shinkoumi who lives his life in a carefree and apathetic manner, showing no concern for his future. The only thing that captures his interest is painting—more specifically, painting the innermost desires of other people.