Touhou Mahoujou: Book of Star Mythology

The story was written by Milka Oyasu (the writer for The Last Comer, Mystical Power Plant and Riverbed Soul Saver).

In an effort to gain more faith, the Moriya Shrine had decided to take the Tanabata featured in the outdated Lunar Calendar (which had Tanabata being held during August as opposed to July) and market it as a new "Moriya Tanabata", using the common morning glory to advertise it.

As a result, an overwhelming number of human wishes became divine spirits, causing the River of Heaven to open up again and reunite Orihime and Hikoboshi outside of schedule.

Soon afterward, a torrential rain began to fall, bringing the premature Tanabata to an end. This rain, combined with the new Tanabata amplifying the stars' power, prompts Reimu, Marisa, and Sanae to investigate.