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!?