Hidden Star in Four Seasons/Story
This article describes the story of Hidden Star in Four Seasons in detail.
There exists a secret god known as Okina Matara, who lives in Ushirodo-no-Kuni (The Land of Rear Doors) along with her two servants, Mai Teireida and Satono Nishida, who Okina wishes to find replacements for. With that in mind, she orders her servants to put doors on the backs of other people and empower them, with the intent of bringing out their full potential and find out if they're worthy of being Okina's servants (While Okina was being genuine about finding new servants, she really just wanted to make a show of herself so she could be remembered by everyone in Gensokyo). But when Satono and Mai started empowering Gensokyo's fairies, their doors began emitting seasonal energy, which made the seasons go out of whack, thus beginning the incident.
Despite it being midsummer, the seasons in various locations are completely out of whack. The Hakurei Shrine is showered in cherry petals, the Youkai Mountain is basking in the middle of autumn, and the Forest of Magic is blanketed in snow. And thanks to some strange force, Gensokyo's fairies are running rampant with unfathomable strength. In the midst of this undeniable incident, the girls set out to investigate and find the perpetrator behind it.
In the midsummer Skies of Gensokyo, the heroine encounters Eternity Larva, who was one of the fairies who went berserk from the incident. Eternity challenges the heroine, but she is quickly defeated and the heroines advance. The heroine then travels to Youkai Mountain, which is drenched in autumn, where she meets up with a yamanba named Nemuno Sakata. Nemuno didn't approve of the heroines trespassing in her territory, so she tries to chase them off with a danmaku battle. Of course, she's defeated as well. The heroine then travels to the Hakurei Shrine, where she's greeted by a sea of cherry blossom trees, another appearance from Lily White as the stage's midboss, and a komainu named Aunn Komano. Aunn was visiting the shrine for a flower-viewing, but she got caught up in the incident and was ultimately confronted by the heroine. Aunn tried to put up a fight, but because of her weak stature, she was defeated in the end.
When the heroine travels to the wintry Forest of Magic, she meets up with Narumi Yatadera. Narumi was originally a Jizo statue, but thanks to the overwhelming energy that the forest was emitting, she started to come alive as a magician. While Narumi did take notice of the seasonal energy that radiated throughout the forest, she was unaware that she was the source of it. Having been goaded by Mai Teireida (while also wanting to test her newfound powers), Narumi then challenges the heroines to a battle. When Narumi is defeated, she and the heroine then notices an open door that was placed on her back. This door leads to Ushirodo-no-Kuni, which is the residence of the true culprit, so the heroines (save Cirno, who enters the door on her own back) travel into the door in order to find out who's behind the whole incident. After the incident, Satono and Mai continue to serve their master.
So now the heroine has arrived in Ushirodo-no-Kuni, where she encounters Mai Teireida and Satono Nishida, the servants of the final boss. It's revealed that the duo were ordered to put doors on the backs of everyone in Gensokyo and draw out their true potential. What the servants don't know is that they're not serving for allies like they were originally told, but for their own replacements. When Mai and Satono are defeated, they travel deeper in the foreign realm to meet up with Okina Matara. Okina claims that she was trying to find successors for Mai and Satono, but really, this was but an excuse to show off and make everyone remember her. Okina and the heroine battle it out using the power of the seasons; however, since the seasonal power was originally Okina's to begin with, Okina takes the heroine's power and forcibly releases it. This trap results in the heroine being forcibly ejected through the door on her own back, taking her back to Gensokyo.
Reimu's Good Ending
Reimu's Bad Ending
Because the heroine was expelled by Okina during the battle, she was unsuccessful in truly defeating Okina in the main game. Therefore, using the boundary of the four seasons, the heroines travel through the back door in order to find Okina and defeat her for real. After being greeted by Mai Teireida and Satono Nishida, they encounter Okina Matara, who has agreed to not use the seasonal energy she obtained from the heroines and show her true form (which is no different from the form she's usually in, a mish-mash of various traits from the other gods she embodies). Though Okina did put up a fight, she didn't expect the heroine to return to her realm, and thus, she was defeated.