No foreword. But I will give the main titles :

Sunny and Jessica are in one family. They have a corporate named Lee-Jung and Taeyeon and Tiffany are also CEO of Hwang-Kim.

They all enter SM. Sunny came last because of visa problems. Taeyeon soon fall in love with Sunny but she didn't notice and keep doing her other job, as a CEO and also, a mafia in the underworld.

Jessica have an illness but she hide it and only Sunny knows. Jessica and Tiffany have a relationship for a long time and they are officially lovers. But they usually fight, because of differences of theirs manners and their company's problem.

The story take places at the time when they haven't debuted yet.

YoonYul and SooHyo will also take places in this story, but less.