Junior risk analyst Seth Bregman (Penn Badgley), his more senior colleague Peter Sullivan (Zachary Quinto), and trading desk head Will Emerson (Paul Bettany) watch as human resources staff of their (never-named) firm, along with building security, conduct an unannounced mass layoff right on their trading floor, at the start of an otherwise normal business day. One of the fired employees is Peter and Seth's boss, Eric Dale (Stanley Tucci), head of risk management on the floor. Dale attempts to tell his now former employer that the firm should look into what he has been working on, but the contracted human resources staff have no interest other than him quickly leaving the building. While Dale is being escorted out, he gives Peter a USB memory stick with a project he had been working on, telling him to "be careful" just as he boards the elevator.
That night, Sullivan finishes Dale's project and discovers that current volatility in the firm's portfolio of mortgage-backed securities will soon exceed the historical volatility levels of the positions. Because of excessive leverage, if the firm's assets decrease by 25% in value, the firm will suffer a loss greater than its market capitalization. He also discovers that, given the normal length of time that the firm holds such securities, this loss must occur. Sullivan alerts Emerson, who calls floor head Sam Rogers (Kevin Spacey).
These now key employees remain at the firm for a series of meetings with progressively more senior executives, including division head Jared Cohen (Simon Baker), chief risk management officer Sarah Robertson (Demi Moore), and finally CEO John Tuld (Jeremy Irons). Cohen's plan is for the firm to quickly sell all of the toxic assets before the market learns of their worthlessness, thereby limiting the firm's exposure, a course favored by Tuld over Rogers's strong objection. Rogers warns Cohen and Tuld that dumping the firm's toxic assets will spread the risk throughout the financial sector and will destroy the firm's relationships with its counterparties. He also warns Cohen that their customers will quickly learn of the firm's plans, once they realize that the firm is only selling the toxic securities.
They finally locate Dale, who had been missing after service to his company phone was deactivated. He has been persuaded to come in with the promise of a generous fee and the threat of having his severance package challenged if he didn't. Meanwhile, it is revealed that Robertson, Cohen, and Tuld were aware of the risks in the weeks leading up to the crisis. Tuld plans to offer Robertson's resignation to the board and employees as a scapegoat.
Before the markets open, Rogers tells his traders they will receive seven-figure bonuses if they achieve a 93% reduction in certain MBS asset classes in a "fire sale". He admits that the traders are effectively ending their careers by destroying their relationships with their clients. Meanwhile, Robertson and Dale sit in an office, being paid handsomely to do nothing for the day; Robertson vigorously defends herself that she warned of the risks although perhaps not loudly enough. Emerson keeps on closing the positions, but his counterparties become increasingly agitated and suspicious as the day wears on. After trading hours end, Rogers watches the same human resources team begin another round of layoffs on his floor. He confronts Tuld in the executive dining area and asks to resign, but Tuld dismisses his protests, claiming that the current crisis is really no different from various crashes and bear markets of the past, and that sharp gains and losses are simply part of the economic cycle. He persuades Rogers to stay at the firm for another two years, promising that there will be a lot of money to be made from the coming crisis. Rogers notices Sullivan meeting with Cohen; Tuld informs Rogers he will promote Sullivan.
In the final scene, Rogers is shown in his ex-wife's front lawn late at night, burying his dog that has died of cancerâthinking that since the dog had spent most of its life there that it should be buried there. His ex-wife comes out and reminds him that he doesn't live there anymore. She reassures him that their son, who is implied to also work on Wall Street, took a hit from the day's trading but will be okay. As the credits roll, Rogers continues to dig.

Question: Who will learn of the firm's selling of toxic assets?
Answer: Their customers.