Albert Einstein's peaceful summer of 1939 ends when his old colleague Leo Szilard turns up with a warning: the Nazis could be building a weapon out of an equation Einstein wrote decades earlier. The film traces that equation back to 1905, Einstein's "miracle year," when a three-page supplement to his special theory of relativity produced E=mc², the statement that mass and energy are two forms of the same thing. Because the speed of light squared is such an enormous number, the film shows, even a small mass converts into a huge release of energy, the principle that would eventually drive nuclear weapons. It follows the chain from Szilard's visit and the letter he and Einstein sent to President Roosevelt through the scientific race that followed, then sits with the ethical weight Einstein carried afterward for a discovery he never intended as a weapon. The film treats the physics and the politics as one story, ending on the equation's afterlife in the nuclear world it helped create.