Tully's big revelation is that the title character, who arrives after Marlo's wealthy brother Craig (Mark Duplass) recommends a night nanny service to her, is actually a figment of Marlo's imagination. More specifically, she's the younger version of Marlo herself. (Yes, like that goofy Bruce Willis movie Disney's The Kid.)
This element is kept hidden from the viewer for most of the runtime as we watch the relationship between Marlo and Tully blossom into an intimate friendship. In addition to providing much-needed childcare support and housekeeping assistance, the eerily aloof but kind-hearted Tully also becomes a confidant to Marlo, who speaks with her young companion with pointed candor about her anxieties, passions, and disappointments. During late-night hangout sessions, the two discuss Marlo's relationship with her body, her obsession with the Showtime reality show Gigolos, and her nonexistent sex life with her gamer husband Drew (Ron Livingston). No topic is off limits.
The actual twist arrives late in the film when Marlo and Tully take a trip into Brooklyn from their nameless suburban enclave for a wild night of drinking and revisiting Marlo's old haunts. After the two have a fight, in which Tully reveals that she'll be leaving the night-nanny gig earlier than expected, they drive back home intoxicated and exhausted. Almost colliding with oncoming traffic, Marlo crashes her car and sends it soaring into a body of water. We then see the car underwater and Marlo is alone. Tully is gone. Moments later, she reemerges as a mermaid, swimming to Marlo's rescue like a creature from a Guillermo Del Toro film. It's a surreal touch that Reitman telegraphs with foreshadowing underwater shots and an earlier clip from the Netflix kids series H20: Mermaid Adventures.
Marlo wakes up in a hospital, where her husband Drew reveals to the audience (via some exposition-y dialogue with an administrator) that "Tully" is actually Marlo's maiden name. During a conversation about Marlo's mental health, a doctor also tells Drew that Marlo is suffering from a lack of sleep. In the next series of scenes, it becomes clear that the movie isn't really about a magical, whimsical caregiver who arrives in the middle of the night to fix a family's problems. Instead, it's a mapping of Marlo's interior life, particularly the way she relates to and understands her younger self.