Red Eyes: Kanshi Sousa-han The First Mission

This story takes place three months before the establishment of KSBC. While Fushimi is away, the three remaining members of the Fushimi Detective Agency receive an email from Fushimi instructing them to investigate the affair of a housewife named "Ishiyama Reiko". The client is Reiko's husband, Takahiro, and the couple has a young son together. As the three of them investigate, not everything is as it seems...
(Source: DramaWiki)