Marlo, mother of two children, is pregnant again with a child she had not planned. Jonah, her son, has a development problem that doctors could not diagnose. She uses the Wilbarger protocol to brush it on her skin in an attempt to reduce her sensitivity. When Marlo and her husband, Drew, visit her rich brother Craig, he offers to pay for a nursing one night, but Marlo refuses to receive the gift.
Marlo then gives birth to a little girl she calls Mia, and she gets quickly overwhelmed by physical exhaustion and responsibility. After Jonah’s teacher recommends that he be transferred to a different school, Marlo succumbs and accepts receiving a nurse’s help. That night, Marlo receives Tully’s visit, the night buddy. In spite of an initial embarrassment, Marlo and Tully develop a close friendship relationship over the next few nights.
Tully proves to be an exceptional nanny, cleaning her house and making muffins for Jonah. When Marlo mentions that Drew has a fetish for women in waitress uniforms in the 1950s, Tully puts out the uniform that Marlo had bought earlier and involved in a threesome with Drew. One night Tully arrives at work being visibly disturbed and says she has quarreled with her roommate because she was annoyed because Tully has too many adventures. She impulsively suggests going to town together to drink something, and Marlo accepts hesitantly, so the two go to a bar in Marlo’s old neighborhood in Brooklyn.