We find out who the killer is.
Everyone tries to help when Perry attempts to hurt Celeste, but ultimately it's Bonnie (Zoë Kravitz), Nathan's free-spirited wife, who pushes him down a flight of stairs. The scene intercuts with waves crashing at the beach as Agnes Obel's melancholy "September Song" plays. (Check out other songs from the stellar soundtrack here!) Why Bonnie, though? We'll expect to learn more about her in season two, but in Liane Moriarty's book, she had an abusive father who triggered this reaction.
The first season ends with the women (now called the Monterey Five) at the beach watching their children, and it all seems too happy to be true.