Carl Sagan opens the program with a description of the cosmos and the Spaceship of the Imagination. The ship journeys through the universe's hundred billion galaxies, the Local Group, the Andromeda Galaxy, the Milky Way, the Orion Nebula, our Solar System, and finally the planet Earth. Eratosthenes' successful calculation of the circumference of Earth leads to a description of the ancient Library of Alexandria. Finally, the Ages of Science are described, before pulling back to the full span of the Cosmic Calendar.

Sagan reflects on the future of humanity and the question of 'who speaks for Earth?' when meeting extraterrestrials. He discusses the very different meetings of the Tlingit people and explorer Jean-François de La Perouse with the destruction of the Aztecs by Spanish conquistadors, the looming threat of nuclear warfare, and the threats shown by destruction of the Library of Alexandria and the murder of Hypatia. The episode ends with an overview of the beginning of the universe, the evolution of life, and the accomplishments of humanity and makes a plea to mankind to cherish life and continue its journey in the cosmos. The Cosmos Update notes the preliminary reconnaissance of planets with spacecraft, the fall of the Berlin Wall and the end of apartheid in South Africa, and measures towards the reduction of nuclear weapons.

Feast your eyes on a reconstruction of the mega-city and its amazing buildings. Alexandria, built in the middle of rancid marshland, had become in less than a century, the greatest city of the Mediterranean. An ideal position at the crossroads of east and west, and one that brought her great riches. Huge, modern, gleaming white and cosmopolitan, like no other in antiquity, it created its own identity.
While the beauty of its planning, its great arteries, its temples and its ports would be equalled only by its openness to the rest of the world. And the monumental achievements of its sovereigns, still shrouded in mystery, we'll never cease to fire our imagination for millennia to come.