Okitegami Kyouko no Iromihon

The tenth instalment of the Boukyaku Tantei Series.

The only employee of the Okitegami Detective Agency, Oyagiri Mamoru, receives a sudden threatening phone call. "If you want Okitegami Kyouko back, you'll have to come up with one billion yen." He is the only one who can rescue the lonely forgetful detective. Although he doesn't have the powers of deduction like Kyouko-san does, he does have a "memory" of her. Mamoru begins his investigation with his own hands. Meanwhile, Kyouko-san wakes up in the presence of the murderer—? The pure white Okitegami Detective Agency vs. the jet-black kidnapper! How will this head-to-head battle end?!

(Source: Kodansha, translated)