After day 7, you can meet River upon entering the EC club.
River mistakes you for the sub they are supposed to meet, after which they ask if you're interested to play with them.
If you're not too dominant, you can choose.
Choosing no ends the storyline. Choosing yes leads to a scene after which they ask if you want to join them long-term. You need to have IQ over 2 and an uncaged cock to say yes.
After agreeing, River calls to arrange dates (striptease at EC club date, restaurant date, movie date, kinky EC club date, park date, staying at home date). You can choose to go along with the scenario River describes or to refuse. Refusing adds 5 reversal points; you need 15 to be able to try to reverse the roles. Going along with it takes away reversal points once they reach more than 25.
Once you get more than 75 reversal points and go on at least 5 dates, you can choose to make River your sub permanently.
The storyline can continue until you choose to end the relationship.