In the affluent beachfront community of Monterey, Cal., a suspicious death at a coastal elementary school fundraiser draws attention to the frictions among some of the mothers. Madeline and Celeste, longtime friends and confidantes, welcome newcomer Jane and her son, Ziggy, into their inner circle, much to the chagrin of another mother, Renata. An incident with the children at school causes battle lines to be drawn.
Jane deflects Ziggy's questions about why they moved to Monterey. Madeline gets news that her community-theater production might get derailed, and is outraged over a slight from Renata. Celeste suggests to Perry that they see a counselor. Nathan and Ed meet in hopes of smoothing out the tensions between their wives, and themselves.
Hoping to get under Renata's skin, Madeline organizes a trip to compete with Amabella's birthday party. After another argument, Celeste succumbs to Perry's charms before their first session with Dr. Amanda Reisman, a therapist. Madeline gets called for a meeting with Abigail's guidance counselor. Principal Nippal announces the theme for the Fall Gala Fundraiser. Later, Jane seeks Madeline's help with Ziggy's family tree project, and opens up about her past.
Nathan invites Madeline and Ed to a couples' dinner to discuss a "parenting paradigm" for Abigail. Celeste prepares for a city-council meeting about Madeline's play, and bristles at Perry's concerns about her returning to work. Jane meets with Ms. Barnes, Ziggy's teacher, who suggests her son be medically evaluated in light of further evidence of classroom bullying. While Madeline is confronted by a skeleton from her past, her internet sleuthing reveals a key player from Jane's past instead.
Madeline receives encouraging news about the play from her director, Joseph Bachman, but is left concerned by his newly icy demeanor. Principal Nippal and Ms. Barnes share their conclusions about Ziggy and Amabella with Jane. Celeste has a solo session with Dr. Reisman, who tries to get to the bottom of her relationship with Perry.
Madeline worries about the can of worms she's opened for Jane. Jane confronts Renata. Bonnie tells Nathan about a secret project Abigail has been working on. Meanwhile, Ed and Madeline have a frank conversation about the lack of passion in their marriage.
After yet another fight, Celeste makes a bold move. Before the school's long-awaited fall fundraiser, Madeline deals with fallout from her past, while Jane learns who's really been hurting Amabella at school.