The plot it to get out of the island because the native are deranged. Along the way some things are discovered about the island and it's inhabitants. Actually the plot is moving slowly since the weekly chapter release is only 6-10 pages.
But yeah, Natives sacrificed some students and raped some. FMC got taken and became one of the bitch of the alpha native. He haven't had intercourse with FMC, though. MC got separated from the student group and found another group to which he is currently with. Together with the current group, MC is finding a way to rescue the hostages particularly FMC.