雨が降る前触れとされる『逆さ虹』が日本各地で発生。怪獣との関連を疑うゲントは、怪獣研究の第一人者であり、恩師でもある横峯教授のもとへ。
そこで横峯から、ときに恵みの雨をもたらし、ときに荒神となりすべてを奪い去るという神のような存在、ニジカガチについて語られる。
その時、ゲントのもとに巨大怪獣現出の知らせが届く。
Upside-down rainbows are being sighted throughout Japan. They are said to herald the onset of rain. Gento suspects that a kaiju may be behind this and decides to visit Professor Yokomine, a leading kaijuologist and also his mentor from school. Yokomine tells him about Nijikagachi, a god-like being that can at times bless the land with rain and at times summon utter destruction. That's when Gento receives news about an appearance of a giant kaiju!
