Several months after events of Teito Kitan ~Doushi Tantei no Jiken Memo~.
Kudo was staying at Iwasaki residence in Shanghai with Aya when they were assaulted and Aya kidnapped. Iwasaki tells Kudo that the attack was done by members of "The Legion Party", a secret society that appeared in Shanghai lately. Kudo takes maid Masha as assistant and starts investigation.

[From vndbreview]