In the opening scene, we find Marlo (Charlize Theron) gently brushing her son Jonah's (Asher Miles Fallica) skin as part of a therapeutic recommendation for his autism. After this moment of care, she approaches her husband Drew (Ron Livingston) to remind him of their dinner plans at her brother's house, only to find that he believes her brother harbors resentment towards him.
As Marlo, who is nine months pregnant with her third child, prepares for the day, she takes Jonah and his sister Sarah (Lia Frankland) to school. During the school drop-off, Jonah throws a tantrum, but Marlo manages to soothe him before meeting with the principal, Laurie (Gameela Wright). The principal expresses concern over Jonah's "quirky" behavior and suggests that Marlo consider hiring an aide.
Dinner at her brother Craig's (Mark Duplass) home unfolds with family dynamics at play, as Craig and his wife Elyse (Elaine Tan) hint that a night nanny could alleviate some of Marlo's burdens, which she finds disheartening. When her water breaks unexpectedly, Marlo rushes to the hospital and gives birth to a girl named Mia.
Upon returning home, Marlo faces the overwhelming responsibilities of new motherhood—pumping breast milk, changing diapers, and soothing a constantly crying baby. In a moment of frustration, during a meeting at school where Laurie suggests transferring Jonah to a public school, Marlo loses her temper, believing Laurie's recommendations stem from bias against her son.
That night, after putting the kids to bed, Marlo is visited by Tully (Mackenzie Davis), the night nanny who not only helps with the baby but also engages Marlo in meaningful discussions about motherhood. To her surprise, Marlo wakes up the next morning to find Tully has tidied up her home.
As their friendship grows, Tully bakes cupcakes for Jonah's class, leading to a reconciliatory moment between Marlo and Laurie. However, challenges resurface when Jonah struggles with a loud toilet at his new school, causing a scene that Marlo manages to resolve with creative distractions.
On a lighter note, Marlo and Tully share a laugh about Drew's browser history revealing his unusual interests. This leads to an unexpected and adventurous night where Tully dons a waitress outfit, prompting a playful rendezvous with Drew.
Marlo gradually starts to regain her confidence, attending a birthday party where her revitalized demeanor is noted by family. Yet, Tully expresses her frustrations about her personal life and eventually declares that her time with Marlo has served its purpose, leading to a bittersweet parting.
Their escapade to Brooklyn for some nighttime fun results in a moment of escapism for Marlo. However, after a sleep-deprived incident causes her to crash her car off a bridge, she finds herself submerged underwater, where she envisions Tully as a mermaid sending her off to safety.
In the hospital, Marlo learns from a nurse that her exhaustion has taken a toll on her mental health. A striking revelation occurs when Drew uncovers that Marlo's maiden name is Tully, implying that the dynamic friendship was a manifestation of Marlo’s mind during her struggle.
In the end, after her recovery, Marlo returns home to a new sense of normalcy. As she prepares for the next day, she brushes Jonah’s skin, only for him to assert that he no longer needs it, signifying growth. The film concludes with Marlo and Drew sharing music in the kitchen, embracing the journey of parenthood together, with newfound strength to face their challenges.
