NOTE: This spoiler was submitted by Alex
The movie opens with Molly Bloom (Jessica Chastain) narrating her life story in voiceover. She explains that it's true, but she's changed all of the names except her own. We see her at the Olympic skiing trials, the most important day of her life. Her father Larry (Kevin Costner) is there watching her. He was extremely tough on Molly growing up - in flashbacks we see how he pushed his three children to be excellent in both academics and athletics, even being cruel and demanding. As Molly begins her qualifying run, she has a freak accident: a pine bough officials lay down for visibility has frozen over so hard that it unsnaps her from her skis and she crashes horribly. She lays on the ground, unconscious.
The movie jumps to the present timeline. Molly is sleeping in her apartment in LA, copies of her book she's written in tow when the FBI arrests her for running an illegal gambling ring. She tells them she hasn't run a game in two years.
Back to the flashbacks - after her skiing accident Molly moves to LA, wanting to take some time off before going to law school and have some fun. She begins cocktail waitressing and catches the eye of producer Dean Keith (Jeremy Strong), who finds her smart and capable. He hires her as his assistant. One night, he asks her to organize a poker game for him. Molly gets food and drinks and is surprised when the poker players include incredibly famous actors like Player X (Michael Cera), business moguls, and some of the richest and most powerful men in the world. They're playing with tens of thousands of dollars. After the game, the players tip her thousands, and Molly realizes how lucrative a career this could be.
In the present day, Molly is in New York for her arraignment and meets with attorney Charlie Jaffey (Idris Elba). He feels bad for her but thinks her case is a mess - the government is accusing her of having something to do with Russian mobsters. Molly explains they were only players in her game; she wasn't involved with them on a deeper level. He doesn't necessarily believe her - not to mention she can't afford to pay him just yet. The government has seized her five million dollars, and she has no way of collecting her 2 million in debts she's owed out on the street. She's hopeful selling her book will make her able to pay him. He's skeptical but agrees to help with her arraignment, and then refer her to someone else - but at her arraignment, when he realizes the reason she never sold her street debts was that she couldn't be sure how the new debt-holders would collect, he takes her on as a client.
Back to flashbacks, Molly starts making money running the games, and her lifestyle improves as well as the quality of the games. She scouts and vets new players provides drinks and food and runs the bank. Eventually, Dean Keith insists Molly is making enough money running the game, and he's going to stop paying her to be his assistant. But he tells her she has to keep being his assistant, or he will take the game from her. Molly instead takes the game herself, moving it to a fancy hotel and putting more money into making it an incredible event. Eventually, Player X reveals a real cruel streak - he, by his own admission, enjoys destroying the other players' lives. When Molly confronts him about it, he steals the game from her, moving it to another location and cutting her out. Molly thinks about quitting while she's ahead - but she doesn't want to stop, and moves to New York to start a similar high-stakes game there. The game becomes successful in New York, but eventually, when players start reneging on debts, Molly begins taking money from the pots - breaking the law - to be able to protect herself. She also begins using drugs to stay awake at all hours.
Italian mob members approach Molly about taking a cut in exchange for collecting debts - she declines, and they send a goon to her hotel who brutally assaults her. She lucks out when a mob bust happens shortly after that. But eventually, she gets a call that her game has been raided by the FBI. She didn't know several of her players were in the Russian mob. That day she quit running the game and didn't do another for two years.
Which brings us back to present day...
Molly and Charlie meet with the prosecutors. They want all the information on the mob dealings, but Molly doesn't have any. They don't believe that she didn't know - Molly explains that at the end of the game she was addicted to drugs and had gotten sloppy at her job. Charlie gives an impassioned speech arguing for Molly's moral character and her innocence. Eventually, the prosecutors reach a deal with Charlie, which he presents to her: immunity, and her fortune returned if she turns over her hard drives. Molly refuses since the contents have nothing to do with criminal activity, but her texts messages with the players have information that will tear families apart and ruin lives. Charlie desperately wants Molly to take the deal, but she refuses.
Molly goes ice skating to clear her head, and her father finds her. He's come for her trial. They have a heart to heart discussion - Molly wants to know why he was always so cruel to her, why he never liked her as much as her brothers. He admits that he knew Molly had discovered he was having an affair, and his shame about it was what led him to be cruel. He admits how much he loves her and he wants to protect her. Molly refuses to turn over her hard drives and pleads guilty in court - but the judge finds the sentence recommended by the prosecutor to be far too harsh, and instead sentences her to community service. Molly celebrates with her family, unsure what to do with her sudden new lease on life.
We return to the opening scene, Molly having just crashed horrifically in the ski trials. Her father rushes to her side, and she awakens...and declares she's fine and begins to stand up. The ski commentators can't believe what a recovery she's making. In her narration, Molly notes that although she's lost a lot and paid consequences, she's learned a lot along the way and that Winston Churchill defined success as the "ability to move from failure to failure with no loss of enthusiasm." The message being: she's going to land on her feet somewhere, somehow.