A lonely college freshman's life is turned upside-down by her impetuous, adventurous stepsister-to-be.
Tracy, a lonely college freshman in New York, is rescued from her solitude by her soon-to-be stepsister Brooke, an adventurous gal about town who entangles her in alluringly mad schemes. 'Mistress America' is a comedy about dream-chasing, score-settling, makeshift families, and cat-stealing.
College freshman Tracy Fishko (Lola Kirke) is having trouble adjusting to college life at Barnard. Barnard College, Columbia University, is a private women's liberal arts college in the borough of Manhattan in New York City.She eventually meets and befriends a fellow student, Tony (Matthew Shear), and even develops a crush on him.
When Tony begins dating another girl called Nicolette (Jasmine Cephas Jones), she feels alone again. On her mother's advice, she contacts her soon-to-be stepsister Brooke (Greta Gerwig) who also lives in New York. Tracy is skeptical as she is 18, and Brooke is 30. Tracy's mother is marrying Brooke's father.
Tracy submits a story for the Mobius Literary Society. They have a unique way of letting candidates know if they have been selected. They barge into their room at night, wake them up, put pie in their face, and make them sing and stuff. Tracy realizes that her story has been rejected when someone else in her dorm is woken up at night and taken through the initiation ceremony.
Tracy is immediately entranced by Brooke and her lifestyle. Brooke has many ideas, one of it being for a TV show based on a woman who is a government worker by day and a self-invented superhero at night, called Mistress America. Brooke is full of life and energy. She has many friends, which is very different to Tracy's life.
After spending a whirlwind night with her, she pens a short story again and submits it to her college's prestigious literary magazine (The Mobius Literary Society). The story is titled Mistress America, as Tracy sees Brooke as the superhero that she mentioned.Tracy continues to spend time with Brooke who shows her plans for a small and eclectic restaurant called "Mom's" after her dead mother which is being financed by her partner. Brooke surrounds herself with positive people in life. She tells Tracy to start her own literary club.
Brooke suggests to Tracy that there is a lot more money in SAT tutoring, but for that you need to get a high score in the SATs. One night, Brooke is approached by Anna (Rebecca Henderson) at a bar. Anna studied with Brooke in high school and says that Brooke really hurt her feelings with her pranks. Anna calls Brooke mean and Malevolent. Brooke says that she simply doesn't care, and says Anna is the bad one for hanging on to a grudge for so long.
Upon returning home one night however, Brooke finds herself locked out of her apartment and discovers that her boyfriend has withdrawn financial support presumably because he saw a picture of Brooke kissing a musician. With massive bills for the restaurant coming due ($20,000 to the contractors, refrigerator for $5000 and Key Fee Payment of $50,000), Brooke visits a psychic with Tracy. Tracy interprets the psychic's words (that she had to seek out an old friend who hurt her) as meaning that Brooke should ask for the money from her former friend Mamie-Claire, whom she fell out with after Mamie-Claire stole her idea for a T-Shirt business and married her former fiance Dylan. Brooke's idea was to make printed T-Shirts with flowers in all kinds of shapes like skulls and so on.
Tracy has Tony drive her and Brooke to Mamie-Claire's (Heather Lind) home in Connecticut with Nicolette joining them to make sure that Tony and Tracy are not having an affair. Brooke tells Tracy that she should be the one with Tony and not the controlling and possessive Nicolette. Tracy started to see Brooke as someone who was falling apart and was at the verge of a total failure.
At Mamie-Claire's home Brooke and the others crash Mamie-Claire's book party and Brooke pitches her restaurant to Mamie-Claire, hoping she will invest. Mamie-Claire, who confides to Tracy that she did steal Brooke's t-shirt idea to become rich, insists that she will have to talk to her husband Dylan (Michael Chernus). While waiting for Dylan's arrival, Brooke takes a call from her father, who informs her that his wedding with Tracy's mother has been called off.
When Dylan comes home, rather than dismiss Brooke, he asks her to pitch her restaurant. Brooke stumbles, but Tracy, still enthused by the project, helps her to pitch it.
Dylan tells Brooke he will give her the money but insists that rather than invest in it, he will give it to her to cover the debt she will have from dissolving the business. Seeing that Tracy is upset by this, and also the toxic effect the offer is immediately having on Dylan and Mamie-Claire's relationship, Brooke refuses the money.
Meanwhile Nicolette, who believes that Tracy and Tony are sleeping together, confronts Tracy about her story. The entire party reads it, and Brooke becomes offended by the brutal way in which Tracy characterized her. She informs her that they are no longer about to become sisters as their parents no longer want to marry and tells her she will sue. Upset, Tracy, sitting outside the house, takes a hit on the bong that Tony had earlier fashioned from an apple with her initial emblazoned on it, smoking marijuana from Dylan's freezer.
Tracy's story is accepted by the university's prestigious literary society and for a while she joins them. Still finding herself unable to fit in, she decides to start her own literary club, inviting both Tony and Nicolette to submit.
Finding herself alone on Thanksgiving, Tracy goes to Brooke's old apartment and finds her packing her things, about to move to Los Angeles. She learns that Brooke was able to cover her debts, as Mamie-Claire gave her what would have been her share of money from the T-shirt business. Brooke also tells her that she has passed the SAT and that she has been accepted into college and is considering going. Tracy invites Brooke to have Thanksgiving dinner with her. The two eat out at a restaurant, with Tracy, as narrator, musing: "Being a beacon of hope for lesser people is a lonely business."