THE MELODY

The lore is about Ichika, Minori, Kohane, Tsukasa and Kanade get stuck in a strange and unknown SEKAI, the SEKAI is called "Lost Tunes Sekai". They have to do multiple challenges by themselves or together to get the lost tunes of their songs, none of the members in their groups cannot help or enter into the SEKAI. The leaders are forbidden to leave until they find every lost tune of their songs.

Who was the one responsible for this whole mess and why did they do it in the first place?