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 if the giant thing is the legendary giant youkai, daidarabotchi. While Cirno is talking to herself, Sanae Kochiya lands before her, just as the giant thing disappears. Sanae was also wondering about it, believing it to be a robot. She asks Cirno about it. Cirno accidentally mentions the daidarabotchi, but tries to deny having seen anything. Her response makes Sanae suspicious. Sanae defeats Cirno in a match and asks for directions to the giant thing. Cirno states 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, surprising Meiling. 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. Sanae tries to leave but Meiling stops her because this is the estate grounds of the devil where people are not allowed to come and go as they please. After their match, 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 knows that the Geyser Underground Center was created for Kanako Yasaka's plans. It is also the site for the mountain’s technological breakthroughs. Connecting the dots, Sanae concluded that this must be the giant robot's docking bay.
Reimu Hakurei somehow falls inside the Geyser Underground Center. It is implied that Sanae had forgotten to close the cover hatch, leaving the ground above unsupported. When Reimu was walking by, the ground suddenly crumbled, causing her to fall. After recovering from her fall, Reimu asks Sanae where are they. Sanae explains this is the Geyser Underground Center used for researching nuclear fusion. Reimu suspects she is up to no good again and then they get into a fight. Sanae reaffirms that she isn't involved in anything bad. Sanae further explains that the Geyser Underground Center is where the kappa come to research and build. Still feeling the pain from her fall, Reimu decides to believe Sanae, but she warns her that if anything weird happens then it is her fault.
Sanae arrives at the nuclear reactor core, commenting on its extreme heat and how staying too long could be dangerous. The reactor operations had to be halted because Sanae's presence was contaminating the nuclear reactor core. Utsuho Reiuji was sent in to eradicate the "foreign substance" immediately and attacks Sanae. After a spell card, Sanae tells Utsuho that she's a shrine maiden of Kanako but Utsuho doesn't remember who Kanako is. Sanae then tries to explain that she's a servant of higher power than Utsuho herself. Utsuho is confused and explains the reason she came to the nuclear reactor core is because of the notification that a "foreign substance" had contaminated the reactor core. They continue their fight. Eventually, Utsuho forgets what they were doing. Sanae takes advantage of this to tell Utsuho that she was listening to her orders and helping her with the investigation of the reactor. Utsuho asks what she should be doing then Sanae asks her whether or not a "gigantic thing" has been hiding there. Utsuho asks what it is. Sanae answers that it's humanoid and about 100 meters. Utsuho states she has not seen such a thing.
Sanae was thinking to herself in the nuclear reactor when Suwako Moriya jumps down inside and accidentally lands on Sanae. Sanae explains that she arrived here in pursuit of a "giant shadow" which she believes to be a giant robot in Gensokyo. Suwako states she came here to restart Hisoutensoku. She makes the connection that the "giant robot" Sanae saw is probably Hisoutensoku. Sanae learns that Suwako had Hisoutensoku built without Kanako’s knowledge and points out that Suwako has been doing a lot of weird things without telling anyone. She reminds Suwako about how keeping the nuclear fusion reactor a secret in the past made everyone angry. She then decides Suwako has gone too far with selfish deeds and challenges her to a battle. Suwako obliges, and the two engage in a fierce battle.
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.