Brittany Forgler, portrayed by Jillian Bell, is a spirited 28-year-old navigating life in the vibrant city of New York, sharing an apartment with her ambitious roommate, Gretchen, played by Alice Lee. Dreaming of social media stardom, Gretchen constantly creates Instagram videos, asserting that anyone can live a lavish lifestyle regardless of their circumstances, and hopes her content will eventually go viral.
Meanwhile, Brittany juggles her role as a greeter at an off-Broadway theater, embodying the life of a party girl. Unfortunately, her struggles with weight and Adderall abuse come into sharp focus when her boss, Jennifer Dundas, who plays Shannon, expresses frustration over Brittany's tardiness, which forces her to double up on responsibilities.
In search of a prescription for Adderall, Brittany consults Patch Darragh who portrays Doctor Falloway. The visit takes an unexpected turn when he informs her that to improve her health, she must shed some pounds—between 45 to 55 to be precise. As she learns that her BMI is 32, the need for a lifestyle change becomes glaringly obvious.
Inspired by a fit neighbor named Martha, Brittany attempts to join a gym, but soon realizes the membership fee is far beyond her reach, forcing her to hit the pavement instead. Nervous about her running journey, she bravely sets out and manages to run one block. This leads her to an unexpected camaraderie with Michaela Watkins, who plays Catherine, a woman Brittany initially resents for her seemingly flawless existence. However, their interactions reveal that Catherine has her own demons, having battled heroin addiction at a young age.
As Brittany dives headfirst into a night full of debauchery with Gretchen, she eventually joins the running group, where she meets Seth, played by Micah Stock. Learning that they share similar health goals, Brittany’s commitment to weight loss solidifies following a successful group run. With her ultimate aim set on participating in the prestigious New York City Marathon, she and her newfound friends bond over this shared ambition.
Catherine reveals that she had trained for the marathon years prior but was sidetracked by an unexpected pregnancy, while Seth amusingly relays tales about the struggles runners face, including one particularly unfortunate misadventure involving his cousin. As a previous participant, Catherine has a guaranteed spot, prompting Brittany and Seth to try their luck in the marathon lottery.
In her pursuit of fitness, Brittany takes on a second job as a pet sitter, where she crosses paths with Utkarsh Ambudkar as Jern, who becomes an unexpected ally in her transformation journey. However, her shifting lifestyle strains her friendship with Gretchen, ultimately leading Brittany to move in with Jern.
As Brittany inches closer to her weight loss goal, obstacles start to mount. She faces disappointment when she is unable to secure a marathon spot through the lottery, and when Catherine offers to donate for her entry, pride prevents Brittany from accepting. This setback sends her spiraling back into old habits, exacerbated by a painful injury that threatens her running future.
Despite the challenges, Brittany’s journey brings her closer to her roots as she visits her sister, Cici, portrayed by Kate Arrington. Demetrius, who serves as a father figure, helps her regain perspective, leading to her eventual return to New York and a renewed commitment to health.
Fast forward a year, and Brittany finds herself at the starting line of the marathon. As she pushes through the grueling course, she encounters encouraging faces, including Seth, Catherine, and Jern, who professes his love for her. This surge of support propels Brittany past her physical limits, allowing her to cross the finish line.
Now living with Jern, Brittany enjoys a blossoming relationship but grapples with the idea of commitment. With a kiss, she embarks on another run, illustrating her journey of self-discovery and the strength she's gained through resilience.
