Physicist Brian Cox sets out to answer a question that sounds simple and isn't: what time is it, actually? He travels to Mayan pyramids in Mexico, where astronomer-priests built temples aligned to track time and discovered that a day is never a clean 24 hours, a fact that still complicates how clocks are set today. Cox visits the people responsible for defining official time itself, keepers of the atomic clocks that anchor global timekeeping, to see how seconds are actually manufactured and agreed upon. From there the film moves into physics proper: string theory's attempts to describe time at the very beginning of the universe, and Einstein's insight that space and time are the same fabric, distinguished mathematically by nothing more than a minus sign. Cox uses that equation to explain why someone traveling near the speed of light experiences time passing differently than someone standing still, building toward the claim that our everyday sense of time flowing steadily is a construction, not a fixed feature of reality.