Moe Berg, portrayed by Paul Rudd, is a seasoned baseball player with 15 years of experience who makes a drastic shift to serve as a spy during World War II, aiming to outpace Nazi Germany in the critical race to develop the first atomic bomb. In the year 1938, the Germans achieve a groundbreaking milestone by successfully splitting the atom, which ignites a fierce competition. Leading the German effort is Mark Strong as Werner Heisenberg, a distinguished physicist and Nobel laureate tasked with the bomb's creation. In a strategic response, the United States dispatches Moe with the covert mission to eliminate Heisenberg.
Back in 1936, Moe is a member of the Boston Red Sox, showcasing his keen instincts on the field. His coach, Shea Whigham as Joe Cronin, strongly urges him to retire, while Moe's teammates harbor suspicions about his sexuality. Amidst the complexities of his professional life, Moe is secretly involved with Estella Huni, played by Sienna Miller, leading to a clandestine relationship that leaves Estella frustrated by Moe's reluctance to publicly acknowledge their bond.
During a tour of Japan, Moe encounters Hiroyuki Sanada as Kawabata, engaging in a sexual liaison while also documenting Japanese installations on film. The situation escalates in December 1941, when the U.S. formally declares war on Japan following the devastating Pearl Harbor attack.
Moe seeks out OSS Director Bill Donovan, depicted by Jeff Daniels, presenting his footage of Japanese sites. Despite Moe's evasiveness about his sexuality, Bill recruits him, introducing him to key figures like Guy Pearce as Robert Furman and Paul Giamatti as Samuel Goudsmit, who reveal the details of the German atomic bomb initiative and Heisenberg’s involvement. With Heisenberg's whereabouts uncertain, they learn that the only lead is Professor Edoardo Amaldi, played by Giancarlo Giannini, located in an Italian town facing imminent capture by Allied forces.
As they make their way to Italy, Samuel shares his encounters with Heisenberg, who was then striving to preserve German science amidst the oppressive Nazi regime. Samuel expresses doubts about Heisenberg's loyalty, despite his undeniable capability to create the bomb. After the trio successfully rescue Edoardo from retreating German troops, he suggests that if Heisenberg had the intent to pursue the bomb's completion, he would have already done so. In a twist, despite British intelligence downplaying the German bomb's existence, Bill orders Moe to proceed with the assassination of Heisenberg.
Moe contemplates the possibility that Heisenberg may be covertly aiding the allies but is ultimately overruled. Samuel learns of an upcoming conference in Zurich that Heisenberg will attend, invited by his acquaintance Tom Wilkinson as Paul Scherrer. Moe is discreetly smuggled into Zurich and invited to a party where Heisenberg is expected. Sensing Moe's true intentions, Heisenberg deduces that he is an American operative and confronts him on a bridge. However, after assessing Heisenberg's lack of intent to aid the Nazis, Moe makes the crucial decision to let him escape.
In the end, it becomes clear that the anticipated German bomb never existed, and Moe is offered the Presidential Medal of Freedom, a prize he chooses to decline. After the war, Heisenberg voluntarily discloses the locations of Germany’s heavy water reserves to the Allies. Despite the unresolved emotions of Moe and Estella, who marries in 1945, Moe remains a bachelor, forever affected by the choices made during tumultuous times.
