The party, consisting of
Alchemist Fagott aka the only male whose D is very desired by all the girls.
Clarinetta the princess who's here because she want the D but too tsundere/shy about it
Ocarina, dwarf-mixed blood? mage and technically servant of Clarinetta who also don't mind getting the D
Sax, a former succubus (now human) who rather open about wanting the D
The group were hit by an incubus curse, ex-acquaintance of Sax, that cause the females to basically de-leveled and the only way to regain strength is through sex. (as a sick joke on the incubus side, to make them have to fuck someone they might not like to get power back)
They're joined by Harp, a supposedly human nun who's possessed by the angel that Fagott fought in MWO2, and used illusion spell to avoid having to fight any further, but said illusion apparently showed her getting gangraped and fell to pleasure. The angel apparently became unable to return to heaven unless Fagott can teach her, or rather the host what is love (baby don't-)
( I think it's implied that Harp isn't so much a human possessed by angel but rather the angel herself taking human form cuz wanting Fagott's D)
So Fagott, the only D around and not affected by that restriction, is to level through normal means then shoot all that level into a chosen girl of your choice.
There's love value and all that similar to MWO2 but now with all 4 girls, and the game hint that the girls who's not chosen can/might be sent out to sex other people.
Although if your love value is too low, they'll lock their room (this is the case with Harp initially)