After a particularly vicious attack from Perry, Celeste decides to leave him.
Jane finds out from Ziggy that one of the twins (Max) has been hurting Amabella. She tells Madeline and then later Celeste.
Renata husband threatens Jane with a restraining order while she's at the cafe with Madeline and Tom kicks him out. He tells Jane that he has been trying to impress her for a while, and she finally realizes that he is not gay and takes him to the gala.
While they are about to go to the gala, Perry finds out that Celeste rented an apartment and is going to leave her. On the way to the gala, he tries to convince her to stay but she has had enough. There are then interrupted by Renata and Celeste takes advantage of this to get away from him and goes inside.
Ed finally shaves for the gala.
At the gala, Celeste tells Renata the truth about Max hurting Amabella.
While Ed is singing about her (he suspects that she cheated when she keeps looking at Joseph?) she rushes out in tears followed by Jane. She tells Jane that she cheated. Renata then walks to them and apologizes to Jane for everything she put her through and she accepts. Madeline also praises her for apologizing. Celeste joins them with Perry rushing behind her. He asks her to come with him but Celeste repeatedly says no with Renata asking Perry to give her some space. Jane then realizes that Perry was her rapist with both Madeline and Celeste too realize by the look on her face. Perry comes at the girls. Flash forward police sirens. We find out that Perry was the one dead. The girls tell the cops that that he starting hurting Celeste and fell down the stairs. Although the female detective doesn't believe them.
After Perry's funeral, we see Jane, Celeste, Madeline, Renata and Bonnie having a picnic at the beach then having mended fences. We then flashback to the night of the gala and see what actually happened. Perry lunged at Celeste and starting hitting and kicking with the girls trying to break him off but are unable. Bonnie then rushes from behind and pushes him off the stairs to his death.
Back at the picnic, we see someone looking at the girls through binoculars and then hear a click of a lighter as the credits roll.
While Nicole Kidman is up for a season 2 if they come up with something extraordinary, director Jean-Marc Vallee is not for it, saying that the open-ending ([Spoiler (click to open)]the detective watching over the women) is for the audience to decide about what happens later and he doesn't want to ruin it. The audience can imagine whatever they want.
source 1, 2, 3