It all started with Sanae and her talking about the Outside World. The resident friendly youkai of Gensokyo latched onto the idea of a surprise birthday party, and before dawn the next morning a group was sent to decorate the Hakurei Shrine for a surprise birthday party for Reimu, who most assuredly was not born on that date.
Along the way, however, the party encountered Donguriou, an acorn chasing weak youkai who was beat up by a mysterious tall not-lady. After failing to find the assailant, they continued on their task to the Shrine, where they encountered Orin (who slept through everything—mostly) and her pet cosplaying “zombie” fairies, who almost got everyone in trouble before being placated.
Once inside, they proceeded to strew decorations all over the place, putting many thumbtacks into the shrine’s paper walls, and stringing a “Happy Anniversary!” banner over a passed-out Oni’s horns. Then dawn struck, the partygoers arrived, Reimu was surprised and cake was had by some. There wasn’t enough cake to go around.
Then, however, an uninvited youkai showed up, a man with a bandaged arm seeking Reimu and her assistance. He, too, was beat up by a tall youkai in the night. As a birthday gift to Reimu, the party decided that they would hunt down this mysterious person who liked to punch poor, defenseless mythloric monsters, and set forth towards the hole to the Underground, because who else could the assailant be other than Yuugi, a tall brawl-happy oni who was conspicuously absent from the party?