Spoiler:

According to Miko, there've been cases of people having their minds taken over and losing control of their bodies. Interestingly enough, the people who've been using urban legends are most at risk. Reisen suspects that someone's taking advantage of the urban legend incident to cause this separate phenomenon, which has no connection to either Eirin or Sagume's plans. Miko cited Reimu as an example, so Reisen heads over to talk to her. Reimu felt like she couldn't sleep and found herself walking outside without really knowing why; she then starts to lose control of her body, and... is suddenly replaced with Mokou? Mokou says she was just closing her eyes and meditating out in the forest, when *poof* she was over here suddenly. Then Mokou assumes that Eientei's using her as a test subject for weird experiments or something and so they fight.