Strange+

Kou is searching for his older brother Takumi, who left home and has gone missing in the city. Worried that he's in trouble, Kou decides not to go home until he's found him, only to find him all too easily. Takumi, it turns out, has become head of the Mikuni Detective Agency, and has no intention of going back home. Kou decides that if he can't make Takumi return, then he'll join the agency himself.

Synopsis from Anime News Network