For the shooting of a promotional video, Gravi and Procella are at a resort island! From enjoying marine sports to a beach volleyball showdown, both groups are having their fill of the island and summer!

With the shooting going as planned, all that's left now is free time... when a tragic crime has occurred!

Without so much as a single lead on who committed the crime, Koi, claiming the title "the Great Detective Koikku", has set out to solve this crime!

Will Koikku discover the one and only truth!?

One day, Rui caught sight of the "Tsukiusa." mascots (a familiar sight at the "Tsukino Dorms") walking. These "Tsukiusa."s were apparently looking for something important, so Rui decided to give them a helping hand.

Actually, these "Tsukiusa."s are also the members of the female idol units "Fluna" and "Seleas", who also belong to the same company. These girls, who originally were from the Moon Society, for some reason became "Tsukiusa."!

To be able to return to their original forms, Rui and the girls, embark on a strange and charming journey!