Marlo, a mother of two, is pregnant with an unplanned third child. Jonah, her son, has a developmental disorder that doctors have been unable to diagnose; she uses the Wilbarger Protocol to brush his skin in an attempt to reduce his sensitivity. When Marlo and her husband Drew visit her wealthy brother Craig’s house for dinner, he offers to pay for a night nanny as a baby shower gift, but Marlo rebuffs him.

Marlo gives birth to a daughter she names Mia, and quickly becomes overwhelmed and exhausted. After Jonah’s principal recommends that he be placed in a different school, Marlo breaks down, and she retrieves the contact information for the night nanny.

That night, Tully, the night nanny, arrives at the house. Despite an initial awkwardness, Marlo and Tully develop a close friendship over the course of several nights. Tully proves to be an exceptional nanny, cleaning the house and baking cupcakes for Jonah’s class. When Marlo mentions that Drew has a fetish for women in 1950s diner waitress uniforms, Tully puts on a uniform that Marlo had previously purchased and they engage in a threesome with Drew.

One night, Tully arrives to work visibly distressed, and says that she has had a fight with her roommate due to her roommate’s anger over Tully bringing home too many hook-ups. Tully impulsively suggests going into the city for a drink, to which Marlo reluctantly agrees, and the pair drive to Marlo’s old neighborhood in Bushwick, Brooklyn. While at a bar, Tully suddenly tells Marlo that she can no longer work for her, explaining that she was there only to “bridge a gap” and that she feels that she is no longer needed. While driving home, Marlo falls asleep at the wheel, and swerves the car into a river.

Marlo sees herself trapped underwater, and envisions Tully as a mermaid coming to rescue her. She awakens at a hospital with Drew. A staff psychiatrist approaches Drew and informs him that Marlo was suffering from extreme sleep deprivation and exhaustion. When the doctor asks about their nanny, Drew tells her that he does not know much about her. A hospital clerk asks Drew for Marlo’s maiden name, and he provides it: “Tully”. Tully does not exist; Marlo imagined her from her past self aged 26 as a means to cope with stress. Marlo is visited by Tully one last time in her hospital room, where they decide that they have to stop seeing each other, and part amicably.

Marlo returns home, where Jonah tells her that she no longer needs to brush his skin. She goes to the kitchen to listen to music and prepare the kids’ lunches for the next day. Drew comes in and shares the music with her as he helps her.