The first of a two-part documentary in which Professor Jim Al-Khalili examines how some of history's greatest minds began to uncover the secrets of the universe by using and manipulating light. In the first edition, the presenter considers the significance of Greek mathematician Euclid's landmark discovery that light moves in straight lines. He then moves on to Renaissance Italy, where Galileo Galilei unlocked the potential of telescopes in locating Earth's true place within the cosmos, before assessing the implications of Danish astronomer Ole Roemer's groundbreaking finding - that light moves at a finite speed.