Blake Lively Is a Mysterious Woman of Leisure in A Simple Favor Trailer

It's a parallel-universe Serena van der Woodsen, causing trouble for everyone once again.

Following Blake Lively's not so thinly veiled attempt at deleting her Instagram history to garner press for her new movie, A Simple Favor, exactly what we thought would happen, well, happened. She posted the first — albeit short — trailer for the film on Instagram, revealing a deliciously sinister narrative.

Co-starring Anna Kendrick, A Simple Favor tells the story of a small-town mom blogger named Stephanie (Kendrick) who manages to buddy up to Lively's glamorous businesswoman character, Emily, and soon enough the duo are best friends. (Well, at least that's what the mom blogger thinks.) There are martini parties, walk-in closet escapades, and all the fun stuff you can imagine from the new friends' bonding sessions at a mansion. But then a mystery kicks into overdrive: Who is Emily, really? And why does she go missing under such bizarre circumstances? Or, is she really missing at all? "Everybody has a dark side," Kendrick's character narrates. "Some of us are better at hiding it than others. I'm realizing I don't know her as well as I thought I did."

The film, which follows Emily's disappearance after she asks Stephanie to pick up her son from school, is based on the best-selling novel of the same name by Darcey Bell. So if you want to spoil the movie's ending before it comes to theaters, head to your nearest bookseller.

Also starring Linda Cardellini and Henry Golding, A Simple Favor will be out September 14.