Six-year-old Maisie (Onata Aprile) lives in a New York apartment with her parents, Susanna (Julianne Moore), a singer in a rock band, and Beale (Steve Coogan), an international art dealer. Susanna and Beale argue with each other viciously and constantly, often ignoring Maisie and leaving her in the care of their nanny, Margo (Joanna Vanderham). Maisie does not always understand her parents’ actions, such as when Susanna hires a locksmith to change the lock on the apartment’s front door to keep Beale out. When they decide to separate, her parents are granted joint custody of Maisie.
After the separation, Margo moves into Beale’s apartment and they get married soon afterwards. In response to the marriage, Susanna impulsively marries Lincoln (Alexander Skarsgård), a bartender whom she barely knows. Maisie bonds with Lincoln, but Susanna quickly grows resentful of how much Maisie warms to him.
Although both Susanna and Beale think they deserve full custody, their desire to keep Maisie is primarily to spite each other, and they are quick to pawn her off onto the other parent when looking after her becomes inconvenient. Maisie’s time is split between staying with each of her parents, though her primary caregivers become Lincoln and Margo as Susanna leaves on a tour and Beale disappears on business trips. With his relationship with Margo disintegrating, Beale eventually decides to move to the UK.
With Beale and Susanna gone, Margo, Lincoln, and Maisie begin to spend time together. While Susanna is supposedly on tour, the three run into her in New York. After an angry confrontation in which Susanna accuses Lincoln and Margo of stealing Maisie from her, Lincoln declares that their relationship is over, telling Susanna that she does not deserve Maisie.
When Susanna suddenly leaves the city, Maisie once again ends up in Margo’s care. Margo takes Maisie to stay in her cousin’s beach house and invites Lincoln to visit, much to Maisie’s delight, and Lincoln and Margo kiss. One night, Susanna stops by the beach house unannounced in her tour bus to pick Maisie up, but Maisie chooses to stay with Margo and Lincoln instead. After shouting at her, Susanna realizes that Maisie is scared of her and that it is in Maisie’s best interests to stay behind.