Brian Cox uses light as a time machine, tracing how the speed of a photon lets telescopes look backward through cosmic history. He starts at Karnak Temple in Egypt, where the pharaohs aligned stone corridors to sunlight, then moves to a Hawker Hunter jet breaking the sound barrier to illustrate speed limits before tackling light's own, unbreakable one. The film walks through Ole Rømer's seventeenth-century measurement of light's speed using the eclipses of Jupiter's moon Io, then scales up to the Andromeda Galaxy, 2.5 million light-years off, its light having left when Homo habilis walked Earth. The Hubble Ultra Deep Field becomes a window onto the universe's infancy and the expansion of space itself. Cox closes at the Burgess Shale fossil beds in Canada, where trilobite compound eyes and the primitive light-sensing cells of Pikaia mark the moment vision evolved and, with it, the Cambrian Explosion that eventually produced eyes capable of looking back at the stars.