Shrine maiden Reimu Hakurei stumbles across curious ruins near the Hakurei Shrine gateway during an ordinary morning walk. Word spreads fast: only one person may enter, and the reward for doing so is substantial. Seven characters fight each other for the right.
The winner steps inside to find Chiyuri Kitashirakawa and Yumemi Okazaki — outsiders from beyond Gensokyo who arrived in their Probability Space Hypervessel. Their motives turn out to be deeply objectionable to whoever just beat six other people to get through the door.