Charles Darwin worked out natural selection without knowing what a gene was, let alone how one worked, and this NOVA film picks up where he had to stop guessing. Biologist Sean Carroll leads the case, using the tools Darwin never had: genome sequencing, embryology, and the fossil record read side by side. Fruit fly experiments show how a single Hox gene mutation can grow a leg where an antenna should be, revealing the genetic switches that reshape bodies over generations. Stickleback fish pulled from Alaskan lakes turn up nearly identical genetic changes to armored ancestors that lost their bony plates after the last ice age, a case study in evolution caught in the act. The film also lines up the human and chimpanzee genomes, showing how a small set of genetic differences accounts for traits as basic as brain size and thumb dexterity. Animation renders DNA switching genes on and off at the cellular level, translating abstract genetics into something visible. The film's argument is that modern genetics does not overturn Darwin, it fills in the mechanism he could only infer from finches and barnacles.