There’s something about Greta Gerwig.

The “Frances Ha” star steals the show in “Mistress America,” her second collaboration with her beau and creative partner, writer and director Noah Baumbach.

This is Gerwig’s fourth time starring in a Baumbach project, all of which tend to be about the same things: identifying societal definitions of success and failure and examining our increasingly technocratic lifestyles — “Must we document ourselves all the time? Must we?” —among other pertinent millenial concerns. “Mistress” is no exception but avoids feeling that way through the sincerity of its characters’ relationships.

“Mistress” follows Tracy Fishko (Lola Kirke), a writer and wordy intellectual who swirls her wine and calls her mother by her first name. She’s in the midst of a lonely first semester at college when she meets 30-year-old Brooke, the worldly daughter of her mother’s fiance and a self-proclaimed autodidact — “That word is one of the things I self-taught myself.”

The first night they meet, Brooke takes Tracy dancing and to a concert where Brooke jumps onstage to sing with the band. Their night together inspires Tracy to write a short story in which the main character is clearly modeled after Brooke. “She lives exactly how a young woman should live,” Tracy reads in a voice-over that carries throughout the film.

It soon becomes apparent that Brooke’s life isn’t as together as she would like it to be. Following Baumbach’s obsession of struggling millenial characters, she works as a spin class instructor and tutor part time, leaning on an overseas boyfriend to pay her rent. She’s also in the midst of an ambitious plan to open a Brooklyn restaurant that would double as a community center. But with her history of abandoned business ideas, like interior decorating, designer T-shirts and apps, it’s not likely this dream will materialize.

Still, Tracy takes to Brooke like an understudy, clinging to her side and saying everything the older girl wants to hear — like calling the Connecticut city “Greenwich gross-ville.”

However, Tracy’s recurring voice-over begins to show how she truly views the fledgling adult. “People could feel her failure coming,” she writes in a particularly cutting line that compares her future stepsister to a rotten carcass.

Although the plot is fairly predictable — there’s never any doubt Brooke will, at some point, learn of the unflattering short story — “Mistress” makes up for it with an especially humorous third act that is set like a stage play and unfolds in one of Brooke’s high school rival’s, Mamie-Claire, home. The scene builds in a way that recalls screwball comedies of the 1930s, with Brooke discovering the short story in the company of Mamie-Claire and her husband, Brooke’s ex-fiance (Michael Chernus) and others.

Tracy and Brooke’s growing relationship balance the humor.Kirke, 22 at the time of filming, initially feels a bit too old to be playing 18, but she handles Tracy with such a deft hand that her portrayal is believable. Tracy speaks with a judgmental lilt that feels genuine to that of a teenager who believes they know everything.

Kirke adds visible self-consciousness to the character that is both necessary and realistic, grounding her so that she always feels human.

Whereas Tracy struggles with the unhappiness that feeling “better than” causes her, Brooke must deal with never feeling “just enough.” Gerwig, the film’s clear standout, embodies the distinctly millennial desire to seem successful and fulfilled. Just as she does in Baumbach’s “Francis Ha,” she jumps between ventures, hoping to make something stick, but nothing does.

Eventually, Brooke’s boyfriend stops paying her rent, forcing her to crash on Tracy’s dorm room floor. The juxtaposition between this scene and Brooke’s life advice tweets — “always be early for work” — are both funny and sad.

Toward the end of the film, Brooke’s ex-fiance tells Brooke that “Whatever you’re doing, it’s working,” referring to her haphazard occupations. Gerwig smiles, but it’s one marred by a deep sense of disappointment. “No, it isn’t,” she admits, both to herself and to any and all millenials in the audience.

Despite the tenuous connection of their parents’ marriage, Tracy and Brooke manage to build something real. Brooke enjoys having a young person to follow and fawn over her, and Tracy, despite herself, enjoys doing the fawning.

They are two lonely people who manage to create a real friendship and appreciation for each other. And when that tenuous connection is pulled out from under them, the bond that’s left is much stronger.