Ah.... Well its not a different story, its a sequel and the characters in side story are important characters in the base story and they do have a history. You do need to read the base story to understand how they ended up in the situation
Still, I think a summery of what happened is also fine. I dont remember their names but here it is:
The bottom in side story was in love with the main character of sadistic beauty but she doesnt love him and she loves someone else. They have a mutual friend (the top) thats in love witht he bottom and assults him. He came to the main character for help and she just ended up assulting him as well. In the end he begs to be with her but she ends up with someone else and the top kidnapped him after he lost everything including his moeny and job. Thats were side story starts. And yeah its basically just stockholm syndrom

would i have to read sadistic beauty main story to understand the bl side stories? or is it basically just an entirely different story set in the same universe