One early morning, a large human-shaped object appeared in the mountains, spewing steam, before disappearing into the mountain fog. Sanae Kochiya, Cirno, and Hong Meiling all saw the mysterious thing. Sanae thought it might be a giant robot; Cirno was convinced it was a daidarabotchi; Meiling believed it was the Taisui Xingjun. Each of them set out to confront the mysterious thing.
Cirno wonders about the giant thing being the legendary giant youkai, daidarabotchi. After talking to herself, Sanae Kochiya lands also wandering about the giant thing who's trying to find it after it disappears. Both Cirno and Sanae have a match and eventually, Sanae wins. After the match Sanae asks for directions to the giant thing. Cirno says she saw it near the Scarlet Devil Mansion.
At the Scarlet Devil Mansion, Hong Meiling is talking to herself about the giant thing, thinking it was Taisui Xingjun. Sanae drops by there, making Meiling surprised. Sanae asks for directions to the giant thing. Meiling says that it headed towards the mountain. Sanae then wonders if she passed it at some point but suddenly, Meiling thinks that Sanae is coming in as she pleases then they have a match against each other. After that, Sanae accidentally blurts out that is a giant robot. A lukewarm mist appears at the Scarlet Devil Mansion and Sanae spots the giant robot at the base of the mountain.
The reappearing human-like giant seems to be rampaging near the Geyser Underground Center at the base of the mountain. Just as she thought that, both it and the mist disappear again. Sanae thought that the grounds under the geyser were strange, so she descended underground to where the sun resides. Sanae explains that the Geyser Underground Center was created for Kanako Yasaka's plans which is the site for the technological breakthru, then thinking that it must be the giant robot's docking bay. Then explaining that in the Outside World, giant robots don't exist outside of anime but if in Gensokyo, then maybe they do.
Reimu Hakurei falls inside the Geyser Underground Center thinking that it's a pitfall trap and wondering why its in her way. She then thinks Sanae is doing something nasty then they get into a fight. Sanae says that she isn't involved in anything bad. Reimu begins to feel some pain in the pitfall. Sanae explains that the Geyser Underground Cenetr is where the kappa come to research and build. Reimu believes that she isn't involved in anything bad but she says that if anything weird happens then its Sanae's fault.
Sanae arrives at the nuclear reactor core, saying that its extremely hot down there and could be dangerous if staying there too long due to high temperatures. Utsuho Reiuji unexpectedly attacks Sanae, ordering that due to the contamination of the nuclear reactor core, reactor operations are to be halted and the "foreign substance" eradicated immediately and Sanae is forced into a fight with Utsuho. After a spell card, Sanae says to Utsuho that she's a shrine maiden of Kanako but Utsuho doesn't understand who Kanako is. Sanae then says that she's a servant of higher power than Utsuho herself. Utsuho explains the reason why she came to the nuclear reactor core is because of the notification that a "foreign substance" had contaminated the reactor core then they continue their fight. Utsuho forgets what they were doing and Sanae says that she was listening to her orders and helping her with her investigation of the reactor. Utsuho asks what she should be doing then Sanae orders to investigate whether or not a "gigantic thing" has been hiding there. Utsuho asks what it is. Sanae answers that its humanoid and about 100 meters.
After waiting for some time in the reactor, Suwako Moriya jumps down inside and accidentally lands on Sanae and she explains that she came inside in pursuit of a "giant shadow" but there was actually a giant robot in Gensokyo. Suwako then says that the giant robot is Hisoutensoku. She says that if it's a "giant robot" then its probably Hisoutensoku and then after a long conversation, they get into a fight.
Cirno speculates she has seen a daidarabotchi. She gets ready to leave when Sanae Kochiya asks her if she has seen a giant human-shaped object. Cirno denies seeing a daidarabotchi, which causes Sanae to be suspicious and fight Cirno. Later, Cirno passes by the Scarlet Devil Mansion when she overhears Hong Meiling mumbling about the Taisui Xingjun. Cirno becomes interested in finding the Taisui Xingjun, but focuses back on the daidarabotchi after beating Meiling.
At the entrance of the Underground Geyser Center, Cirno meets Marisa Kirisame who tells her that the daidarabotchi is inside the Underground Geyser Center, but that was a complete lie. Once inside after beating Marisa, Cirno is intercepted by Utsuho Reiuji. Utsuho attempts to eradicate the "foreign substance", but Cirno manages to escape being incinerated to death. After escaping, Cirno runs into Marisa, who tells her about a giant youkai in the Forest of Magic.
Hong Meiling rushes into the library of the Scarlet Devil Mansion to talk to Patchouli Knowledge about seeing Taisui Xingjun and advises Patchouli to prepare for an earthquake. Patchouli however tells Meiling to go back to her post and not interrupt Patchouli's research. After a short battle with Patchouli, Meiling then decides to defend the Scarlet Devil Mansion from the Taisui Xingjun and his minions on her own.
Later, Alice Margatroid appears and asks Meiling to escort her to the library. However, Meiling claims that she is actually a fake sent by the Taisui Xingjun, and "Alice" is impressed that Meiling saw through her ruse. Meiling then defeats "Alice" and prepares for the next invader. Afterwards, Marisa Kirisame comes by hoping to visit the library again. Marisa tries to get past Meiling, but Meiling ousts her as a fake and defeats her. Reimu Hakurei then arrives at the Scarlet Devil Mansion to exterminate youkai. Meiling recognizes Reimu as yet another agent of Taisui Xingjun and proceeds to defeat the "Reimu".
After she is defeated, "Reimu" decides to show Meiling her true form, a Giant Catfish. The catfish tells Meiling it has finally been revived, and reveals it sent assassins to weaken Meiling before its revival. Meiling decides to fight the catfish. After a harsh battle, it was defeated.