NOTE: This spoiler was submitted by Alex

Brittany Forgler (Jillian Bell) is a funny and well-meaning but overweight woman living in New York City. She shows up to work late and parties all night with her best friend and roommate, social media influencer Gretchen (Alice Lee). She makes a doctor’s appointment in order to get a prescription for Adderall, but the doctor refuses to give that to her, and instead recommends that she try to lose weight and eat healthier because she’s high on the BMI index and has high blood pressure and could be putting her health at risk. She’s offended and taken aback.

She continues partying and being messy, but finds it hollow and empty. She breaks down crying in her apartment, which her upstairs neighbor Catherine (Michaela Watkins) hears and comes down to check on her. Brittany tells her that she’s broke and fat and a failure, and wants nothing to do with Catherine, who she had been calling “Martha” because she was generally scolding Brittany and Gretchen about leaving their mess in the shared hallway. Catherine offers some sincere concern, but Brittany wants none of her pity… she sees Catherine – who is married with two kids and only has the apartment upstairs as her photography studio – as having a perfect life and judging her. Catherine tells Brittany that in her own 20s she was a drug addict, and that her life isn’t perfect. Brittany asks to be left alone.

Later, while watching Catherine out running, Brittany decides to try. She struggles to make it one block, but finishes. When she returns, Catherine invites her to a running group she runs in. Brittany goes to the group and meets Seth (Micah Stock), who has joined after being winded chasing his husband and kids around. The two become fast friends and running buddies, and Catherine begins slowing her pace to hang with the two of them. Brittany doesn’t want her pity and wants to avoid her, but Seth wants to let Catherine hang with them because she’s going through a divorce and needs this right now. Eventually, spurred on by Brittany, the three decide to enter the New York marathon the next year as an end goal.

Brittany begins training hard, losing weight. Gretchen keeps trying to get her to come out and party and drink, but Brittany stays in, trying to eat better and keep her morning run schedule. Gretchen doesn’t take her seriously and says she’s not a “real” runner. In order to enter the marathon if you weren’t already in a running club like Catherine, you need to enter a lottery or raise money for charity. Brittany applies for a second job, eventually finding a job house/dog-sitting for a rich couple with a brownstone and are never there. She meets Jern (Utkarsh Umbudkar), who is the night house sitter who has actually been living in the house during the day as well. He has a bit of man-child syndrome and he and Brittany sometimes get along and sometimes don’t. Eventually, Gretchen and Brittany have a huge fight, where Gretchen tells her she’ll always be a fat girl. Brittany also begins staying full time at the brownstone.

Jern helps Brittany set up her dating profile post-weight loss, and she goes on a date but bails, uncomfortable. When Jern gives her a pep talk, she starts to think she might have feelings for him, and eventually, they sleep together. Seth wins a marathon spot in the lottery, but Brittany doesn’t, so she vows to raise money for a charity – and at Catherine’s divorce/house-warming, Seth and Catherine surprise her – Catherine’s making a large donation in her name, giving Brittany a spot. They tell her they wouldn’t be running the marathon without her. Brittany rejects the gift, telling Catherine she doesn’t need her pity. Angry and alone, Brittany trains extra hard – her ankle is sore but she runs on it anyway until it’s in serious pain. She goes to her doctor who tells her she should have paid attention to her pain because now she’s got a fracture and will be in a cast for weeks – meaning she won’t be able to run the marathon. When Jern comes home and sees her cast, he’s concerned, but she’s rude and dismissive of him and tells him fat Brittany is coming back – and then the brownstone owners come home and discover they’ve been living there. They are both fired and leave. Brittany, devastated, returns home to stay with her sister and her sister’s husband/her surrogate father Demetrius (Lil Rel Howery), who helped raise her after her dad died, in Philly.

The day of the marathon, Brittany ignores Seth and Catherine’s calls, seeing on social media that they completed the marathon. She gets drunk at Demetrius’s birthday party, and when a plus-size woman, Jasmine, and her boyfriend come to the party, Brittany horribly insults them by asking how they can be together and how she can be happy. Jasmine leaves in tears, and Demetrius has a talk with Brittany where he tells her she needs to stop acting this way and start letting people in. Brittany apologizes to Jasmine and heads back to New York, she apologizes to Seth and Catherine for cutting them out, and finds out Catherine’s husband is suing her for full custody after claiming she relapsed, though she explains she was giving a prescription and kept it hidden from him for fear of this. Brittany realizes she needs to be there for her. She also reunites with Jern and tells him she just wants to be friends for now, since she’s a mess and needs to figure herself out. He agrees to her terms.

One year later, Brittany is running the marathon. She makes it very far but begins cramping up and collapses, thinking she won’t be able to finish. But then she hears Seth on the sidelines, cheering her on. She sees Catherine encouraging her. She keeps going, and before the finish line, Jern is there telling her he loves her. She finishes the race and returns home to her apartment where she and Jern are now living together. He floats the idea of proposing and she playfully shoots it down and goes for a run.