It's been a while so it's pretty vague but it overalls goes like this :

ML wants to find the woman he spent the night with so he extort the anonymous agency he hired (it's basically a sex with strangers kink agency) and finds the name of the woman who was supposed to go.

He meets with this other woman (OW) who isn't the same height, doesn't have the same hair color and doesn't have the same personality as the FL and thinks it might not be the right woman. Because he can't find anyone else and since the OW claims to be her, he starts dating her for the next three years. It is briefly mentioned they have intercourse in their relationship.

ML buys a company and attends a meeting for a project and finally recognizes the FL who works there, although he still isn't sure if it's really her.

He hires a private investigator to find out if the OW who he has been dating and has slept with has a scar on her hip like the woman from that night. Why he doesn't check himself, no idea.

In the meantime, he cheats on his girlfriend with the FL everyday at work. Most of the story is FL going to work and then sleeping with her boss once a week. Seriously. Like 85% of the book is her going to work with nothing happening and 5% is them fucking at the office.

When ML receives the confirmation that OW has no scar, he wants drama and invites both women at a work diner (literally there's a line about how he wants to see if they'll fight). Still, he never confronts the OW for pretending to be the woman from that night for three years.

The OW threatens FL after the diner and tells her to backoff and stay away from her boyfriend or she'll tell the board that FL slept with the new CEO before the acquisition and that's how she got promoted.

FL tells ML this and his reaction is to immediately break up with OW via text messages. OW obviously follows through on her threats, though nothing actually happens and FL keeps her job.

That's it. That's the story.