all characters are at least 18
The protagonist (player) is a newcomer in a Production Agency (Manager for Idols basically) for the 5 heroines. One of the heroines (red-haired) is a childhood friend of the protagonist (she calls him "Onii-chan"), and her mother is the boss of the agency. Another heroine in the agency (white-haired) is the protagonist's underclassman in the past so she calls him "Senpai". The other three are strangers.
After H scene with 2 heroine, 3 of the heroines discovered that whoever has sex with the protagonist gets inexplicable luck when it comes to getting job offers, and actual sex improves luck more than foreplay. The word spreads and the rest also have sex with the protagonist. The story is basically the heroines decided to keep having sex with the protagonist for that reason. (The two heroines that knew the protagonist do have feelings for him)
That is where free H mode picks up.