Mysterious monster mansion

Hiroshi and his friends suspect that Matenro's true identity is Ochiai Sensei, and begin investigating Ochiai's background. However, Ochiai is a completely dull man, and even if I'm mistaken, he doesn't seem to be Matenro. Matenro appears in front of the Hadogumi, who are disappointed that their attack has failed. He asks you to come to a Western-style mansion tonight that is rumored to be a "monster's mansion" and steal the treasure, but...