It starts with the MC getting paid (or paying) a witch to train him to get stronger, but all she does is rape the MC and told him he is too weak and he should just give up as an adventurer. The guy then has a flashback to when he was living with his fiance (or childhood friend) as he works as an adventurer trying to make enough money for the both of them. I don't really remember the rest, but it basically involves this high rank adventurer drugging the girl's dad and offering her the cure if she have sex with him, where she slowly gets corrupted until she leaves the MC for the high rank dude.