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 the Land of the Back Door along with her two servants, Mai Teireida and Satono Nishida, who Okina claims she wishes to find replacements for. She orders her servants to draw the latent powers from the doors she placed on the backs of other people in order to 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 may have been genuine about finding new servants, her main goal was to make a show of power, sending a message to the other Sages of Gensokyo and reminding everyone of her existence). When Satono and Mai started empowering Gensokyo's fairies (who are inherently tied to nature), the seasons began to wildly fluctuate all throughout the land, beginning the incident.
Despite it being midsummer, the seasons in various locations unnaturally spiral out of control. 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. Thanks to some strange force, Gensokyo's fairies are running rampant with newfound strength. In the midst of this undeniable incident, the girls set out to investigate and confront the perpetrator behind it.
In the midsummer Skies of Gensokyo, the heroine encounters Eternity Larva, one of the fairies who went berserk due to the incident. Eternity challenges the heroine, but she is quickly defeated and the heroine advances. The heroine travels to Youkai Mountain, which is in the middle 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 through 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, an appearance from the fairy 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 is eventually defeated.
When the heroine travels to the wintry skies of the 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 was given life 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 the Land of the Back Door, the residence of the true culprit. The heroine (save Cirno, who enters the door on her own back) travels into the door on Narumi's back in order to find out who's behind the whole incident.
Arriving in the Land of the Back Door, the heroine encounters Mai Teireida and Satono Nishida, the servants of the final boss. It's revealed that the duo were ordered to draw out the powers of everyone in Gensokyo in order to reveal their true potential so that the culprit could choose more servants. The servants don't know, however, that they're not searching for more allies like they were originally told, but for their own replacements. Mai and Satono are defeated and the heroine travels deeper into the backwards realm. They eventually meet up with Okina Matara, a secret god and sage of Gensokyo. Okina claims that she was trying to find successors for Mai and Satono, but unbeknownst to the heroine, her true motive was to show off her power as a message to the other sages and to remind Gensokyo of her existence. Okina and the heroine battle it out using the power of the seasons but, 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 ejected through the door on their own back, taking them back to Gensokyo and essentially losing them the battle for the time being.
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 during the main game. Thus, using the power of the boundary between the four seasons (doyou), the heroine travels back to the Land of the Back Door in order to find Okina and defeat her for real. After being greeted by Mai Teireida and Satono Nishida once again, the heroine encounters Okina Matara, who has agreed to not use the seasonal energy she obtained from the heroine. She also sports her true form (which is no different from the form she's usually in, a chaotic combination of various traits from the other gods she embodies). In the end, the heroine defeats Okina once and for all (although during Aya's scenario, Okina reveals that she could've beaten the heroine at any point during the battle by closing the door on their back) and the incident resolves.