From Renta!:

Keiji Nishiono, 36, runs a small, unsuccessful detective agency with his only assistant, the beautiful-but-strict Eisei Takatoh. Laid-back and empathetic to a fault, Keiji is constantly being scolded by Eisei, who is much more of a realist. One day, they accept a case about relationship troubles and go undercover as a couple to infiltrate a gay bar...!! Their investigation goes well, but egged-on by the gay men around them, they end up kissing. After that incident, they start seeing each other as more than business partners...