Meitantei Kouko wa Yuutsu

During the Showa era, there was a law in Japan that granted a detective diploma only to those who solved three difficult cases. Kouko, an aspiring detective, receives a request to find out who's threatening to murder Inukami-kun, the son of a prestigious family, who is staying at Shunjukan, a boarding house in Tokyo.

Kouko infiltrates Shunjukan as a cook who takes care of the lodgers, and starts to investigate. However, she's not good at cooking, or reasoning, so she's very busy! What's more, the four lodgers who live there are all crazy...?!

(Source: Hakusensha, translated)

Note: Chapter count includes a prologue and one extra.