A strange phenomenon called "Perfect Possession" is taking place. A phenomenon where one is taken over by a completely different person, not only in mind, but also in body. But have no worry: Reimu is already on her way to investigate the cause behind its occurrence.

"The strongest duo have joined forces."
"Something bad may be in store for us humans."
"An opponent impossible to defeat by anybody alone?"
Ludicrous rumors are circulating, seemingly out of the fear in human villagers' hearts.

The so-called "Urban Legend Incidents", yet to be resolved, have bestowed unsubstantiatable rumors with a tendency to manifest in concrete forms. And this time, the Perfect Possession is obviously yet another Urban Legend Incident.

This story is about those who keep watch on Perfect Possession, those who exploit it, those unwittingly dragged into it, and those working to expose the threat behind it...
Those who have made the mistake of seeing themselves as the strongest duo. This is a bittersweet story about them.