The theory of Némesis, a star that orbits the Sun and causes catastrophic events is explored. Nemesis is a hypothetical red dwarf or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years) somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years.

Professor Jim Al-Khalili continues his investigation of the atom by looking at radioactivity, the Atom Bomb and the Big Bang, and even why we are here and how we were made. He shows that, in the quest to understand the atom, the mystery of how the entire universe was created became revealed

At the dawn of a new century, a new theory is being born. It threatens to demolish the foundations of 20th century physics. Its authors are two of the world's leading cosmologists. If they're right, Einstein was wrong. It all began when Andy Albrecht and Joao Magueijo met at a conference in America in 1996. This program began from Newtonian view of the universe then takes you through the General Relativity and the Flatness problem. This leads to the Horizon problem and its solution, the Inflation theory. However modern astronomy doesn't stop here, the Inflation theory has its flaw too, and what happened before the big bang? This can all be answered by changing one thing, the one thing no body dare to question until now.

A look at the solar system's largest planet, Jupiter; its formation and composition and its mini-solar system of over 60 moons – some of which may have the potential to support extra-terrestrial life.

Ideas about time and space are explored in the changes that constellations undergo over time, the redshift and blue shift measured in interstellar objects, time dilation in Albert Einstein's theory of relativity, the designs of both Leonardo da Vinci and spacecraft that could travel near light speed, time travel and its hypothetical effects on human history, the origins of the Solar System, the history of life, and the immensity of space. In Cosmos Update, the idea of faster-than-light travel by wormholes (researched by Kip Thorne and shown in Sagan’s novel Contact) is discussed.

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.