Seth Davis (Giovanni Ribisi) is a 19-year-old dropout from Queens College who runs an unlicensed casino in his apartment. Although he earns a successful living in his line of work, he is constantly berated by his father, Marty (Ron Rifkin), a New York City federal judge. One night, his old friend Adam (Jamie Kennedy) stops by the casino to play blackjack, bringing a wealthy associate named Greg Weinstein (Nicky Katt) along with him. Although Seth is content with his earnings through the casino, he becomes increasingly unnerved due to Marty's disapproval and the risk factor of having his business shut down along with his arrest. On another night, Greg recruits Seth to join J.T. Marlin, a brokerage firm based somewhere off the Long Island Expressway, "a good hour away from Wall Street" as Seth describes it. He is impressed with not only the chance to earn a legal living, but to make millions of dollars.
Arriving at J.T. Marlin, Seth attends a group interview and learns from Jim Young (Ben Affleck), one of the co-founders of the firm, what is expected of his work and also how he can become a millionaire. The firm's techniques of selling are through cold calling investors to sell stock, and Seth joins as a stockbroker trainee, having to close 40 accounts and pass a Series 7 Exam in order to begin working independently. He finds success in his new job as he proceeds to do very well in his work, as well as impressing his parents, becoming part of a crowd of successful senior brokers, and embarking on a romance with Abbie Halpert (Nia Long), a secretary and ex-girlfriend of Greg. Seth also experiences trials and tribulations, including altercations and arguments he experiences with Greg and the decision to close down his casino.
On the other hand, J.T. Marlin has been under investigation by the FBI, revealing the firm's illegitimacy. The company is a chop stock brokerage firm that runs a "pump and dump", using its brokers to create artificial demand in the stock of expired or fake companies. When the firm is done pumping the stock, the investors then have no one to sell their shares to in the market, and the price of the stock plummets. The federal agents decide to pursue Seth, figuring he is a good target due to his intelligence, and his potential of not being too loyal to the firm due to his short tenure.
Seth passes his Series 7 Exam and becomes a senior broker. He then contacts Harry Reynard (Taylor Nichols), the purchasing manager of a gourmet foods company, different and less privileged than the other "whales" who are common prey for the firm. Although Harry emphasizes that he and his family are being frugal at the moment in order to buy a house, Seth falsely promises a good investment for him that can very well turn out to be highly profitable. After selling him 100 shares at $8. each, Harry calls back to ask why the stock has done so poorly. Seth sells him even more worthless shares, persuading him the outcome will not be consequential. This creates an excruciating financial situation for Harry and his family, causing him to lose the house he had wanted to purchase as well as being abandoned by his family.
Unnerved by the decision to scam Harry, Seth comes to terms with knowing that J.T. Marlin isn't as legitimate as it was made out to be before. Marty then disowns him, blasting him for his dishonest work of stealing and destroying other peoples' lives. Despite this, Seth shows up at his father's office and emotionally explains that he shut down his casino and went along with a highly criminal line of work that he thought was legal in order to gain his family's approval. He then requests that his father helps him on an IPO scheme in order to rob the firm of their money and bring them down. Although Marty initially refuses due to the risk of losing his judgeship, he calls Seth the next day, reconciling with him and offering an involvement in the scheme.
Seth is eventually arrested by the FBI for the violation of 26 SEC and NASD regulations, and is brought into their custody along with his father, as the bureau had discovered their IPO scheme due to a tape-recorded phone conversation. The FBI offer him federal immunity if he agrees to testify against J.T. Marlin once all the suspects are brought into court, but can also involve Marty due to evidence of their intentions for an illegal activity. Seth asserts that he will testify against the firm and provide strong evidence of their illegal practices only if his father is released. He and the agents come to an agreement on this, with Seth being kept overnight only to return to work the next day and make copies of investment files onto a floppy disk in order to use as evidence. After that, it is implied that he will be free to go as the FBI will proceed to raid the building and prosecute everybody else.
Seth returns to work the next day and goes along with the FBI's instruction. Before leaving, Seth attempts to redeem himself for his actions and attempts to get Harry's money back. He lies to Michael Brantley (Tom Everett Scott), the company founder, by explaining that the firm can lose a lot of money by refusing to continue to do business with Harry Reynard, who Seth makes out to be an important and profitable prospect at a make-or-break turning point. Brantley agrees to proceed based on Seth's explanation, offering him shares of the next IPO, however, with a caveat that he cannot sell the shares until the firm has sold off their shares. In order to sell the shares behind Michael's back, Seth needs a ticket sale signed by a senior broker, something that his direct supervisor, Greg, has explicitly said that he would never do.
Seth seeks a signing from Chris Varick (Vin Diesel), explaining that he may well "do one thing right" in helping a severely hit investor make his money back, now that the firm will be raided and, soon enough, there will be no future in continuing business at J.T. Marlin. Chris reluctantly agrees to do so, and proceeds to escape the building in an attempt to flee federal enforcement. Seth walks out to his car, deciding what to do with his life now that his ties with J.T. Marlin are finished. As he departs in his car, several FBI cars, buses, and tow trucks enter the parking lot, with agents storming out ready to raid the building.