Physicist Jim Al-Khalili traces how the study of the atom became the study of the universe's origin, in the second installment of his three-part history of atomic science. The film follows the scientists who kept asking what matter is actually made of and found the answer tangled up with much bigger questions: where energy comes from, why stars burn, and how everything began. Radioactivity opens the story, discovered almost by accident and then pushed toward its most terrifying application, the atomic bomb, built by physicists who understood the atom's structure well enough to split it deliberately. Al-Khalili carries that same nuclear physics forward to cosmology, showing how the same equations that explain fission and stellar fusion also underpin the Big Bang model of the universe's birth. The throughline is competition: the twentieth century's leading physicists racing each other toward the same unknowns, each breakthrough in atomic theory doubling as a step toward explaining how the universe itself came to exist. Archival footage and studio explanation carry the science.