Oppenheimer is told in a nonlinear fashion, focusing on three distinct (but interconnected) storylines: his early life, his time during the Manhattan Project, and his post-war life during which time his security clearance was being reviewed.
The film opens in 1954, when J. Robert Oppenheimer (played by Cillian Murphy) participates in a security hearing related to his wife's Communist-related activities. Admiral Lewis Strauss, played by Robert Downey Jr., has an orchestrated a plot against Oppenheimer, feeling slighted because of his previous disparaging comments against Strauss and because he believed that Oppenheimer was a Communist sympathizer.
In 1926, as 22-year-old Oppenheimer studies for his doctoral degree at the prestigious University of Cambridge in the United Kingdom. Oppenheimer quickly becomes disillusioned with his studies, frequently clashing with his professor. He ultimately becomes determined to poison his tutor's apple, but eventually decides against it after listening to a lecture from professor Niels Bohr, who tells Oppenheimer to study physics in Germany right before the rise of Adolf Hitler. Oppenheimer agrees and leaves Cambridge for Germany.
In Germany, Oppenheimer meets and becomes fast friends with a fellow student named Isidor Rabi. Rabi and Oppenheimer likewise start chatting with German theoretical physicist Werner Heisenberg, who teaches them more about their field.
Then, the film moves to Oppenheimer's teaching career. He begins teaching at Berkley about physics, but attracts only one student for his lecture. After all, few people knew about the emerging field. Over time, though, his classes became progressively more full. And Oppenheimer became a sensation on campus; his class became wildly popular.
Later, Oppenheimer attends a Communist Party gathering with his girlfriend and younger brother. Oppenheimer is largely uninterested in the ideology and most of the people there, but meets and has an affair with a young woman named Jean Tatlock (played by Florence Pugh). They have a torrid affair, which ultimately ends in Tatlock's death under suspicious circumstances. Although nothing else of substance results from their meeting, questions emerge about Oppenheimer's politics, which complicates his life immensely.
Oppenheimer then meets Kitty Puening (played by Emily Blunt), who is an unhappy marriage. They start an affair, and Kitty becomes pregnant with his child. They later marry, but Kitty becomes trapped in a vicious cycle and takes to alcohol to ease her troubles.
Next, Oppenheimer is recruited by General Leslie Groves (played by Matt Damon), to lead up what will become the Manhattan Project to develop the atomic bomb for the United States. Oppenheimer is initially hesitant to join the project, but ultimately agrees after some pressure from his colleagues. He starts to assemble a team of top-flight scientists to help him develop a bomb before the Nazis do and end the war. To that end, they set up a laboratory in the town of Los Alamos, New Mexico, away from prying eyes so that they can conduct their work in private.
Later, Oppenheimer and Strauss quibble at an Atomic Energy Commission meeting, where Strauss says there were Communists on the Manhattan Project worksite, an accusation which Oppenheimer resents. Still, the men discuss the development of the Hydrogen bomb, which would have even more power than the atomic bomb.
Ultimately, Oppenheimer and his team successfully complete work on the Manhattan project. They conduct a test, which they called the "Trinity Test," in which they successfully tested a prototype version of their device, proving their theory and instilling confidence in U.S. leaders that they would be able to win the war. These devices were tested on Japan, a development that deeply troubled Oppenheimer, who feared that U.S. President Truman was opening a proverbial "Pandora's box."
Oppenheimer again faces Strauss, who instigated a review of his security clearance in the late 1950s. During the review, Oppenheimer's statements and activities were put under a microscope. This ultimately led to his security clearance being revoked, something that made him become disillusioned with the U.S. government and the very thing he helped to create, foreshadowing his decision to become an outspoken advocate against atomic weapons.
