In the fall of 2003, Harvard undergraduate and computer programming genius Mark Zuckerberg is dumped by his girlfriend Erica Albright. Fueled by the sting of rejection and a combination of alcohol and coding skills, he creates a site called "Facemash" which allows users to rate the attractiveness of female students. The site becomes an overnight sensation on campus but also leads to disciplinary actions and casts a flare of notoriety around Zuckerberg. Noticing his talents, twins Cameron and Tyler Winklevoss, along with Divya Narendra, approach him with an idea for a social networking site called "The Harvard Connection," aiming to connect Harvard students. Mark agrees to help with the project but is soon consumed by his own idea for a social network.

Mark enlists the help of his best friend, Eduardo Saverin, who provides the seed money to launch "The Facebook," a platform that expands rapidly among Harvard students and beyond. As its popularity skyrockets, the site draws the attention of Silicon Valley entrepreneurs, including Napster co-founder Sean Parker, who manages to get Mark's ear, engendering a vision for a vast global network. This expansion and the involvement of new players strain Mark and Eduardo's friendship, leading to internal conflicts. Meanwhile, the Winklevoss twins learn of "The Facebook's" growing impact, claiming their original concept was stolen, setting the stage for a legal battle over the creation and ownership of the site.