Asteroids strike, planets collide, black holes blast out death rays, volcanoes erupt and ice engulfs the planet. These are the universe's weapons of extinction. They've happened before - wiping out entire species, and they will happen again. Are we next?

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.

The story begins with Tyson sitting at a campfire, and telling how the wolf changed through artificial selection, and selective breeding into the dog breeds around today. He then enters the Ship of Imagination, and explains natural selection with the process that helped to create the polar bears. Along the way he talks about DNA, genes and mutation. Next he goes to a forest and describes the Tree of life, this leads him to discussing the evolution of the eye. He then discusses extinction, by going to a monument called the Halls of Extinction, dedicated to the broken branches of the tree of life. Explaining the five great Extinction events. He then tells how some life has survived, and then focuses on the tardigrade. From there he talks about what other kinds of life might have been created on other worlds. He then goes to Saturn's moon Titan. From there he speculates about life and how it first began. He then returns to Earth and tells about abiogenesis and how life changed and evolved. The show ends with an animated sequence from the original series of life's evolution from one cell to humans.

Right now cosmic forces prowl the universe and threaten man's very existence. They're asteroids and comets; they've left their imprint on planet Earth, literally. Initially helping to build planets through violent collisions, during this fiery bombardment period they may have even seeded Earth with water and the building blocks for life. Since the turbulent formation of the solar system, these space rocks have continued to impact Earth. Some have been so violent that they've led to mass extinctions, including one that wiped out the dinosaur. New theories suggest that asteroid and comet dust harbour deadly viruses that may have triggered some of our worst pandemics. The possibility of future collisions remains a legitimate threat yet despite their dangers, asteroids and comets may hold vital natural resources, which could actually preserve

It sounds like a Hollywood blockbuster: a deadly asteroid is on a collision course with Earth. But in reality, it's only a matter of time before a giant space rock threatens to wipe out civilization. An asteroid took out the dinosaurs sixty-five million years ago. Are we next? This episode analyzes the threat and explores the many ways--from a nuclear bomb to ingenious new technology--that experts are proposing to stop Armageddon

What are the latest discoveries in the deadly world of asteroids? Will a recently returned Japanese spacecraft become the first to bring an asteroid sample back to our planet? What would happen to America's East Coast if the massive asteroid impact that helped form Chesapeake Bay 35 million years ago struck today? And why did President Barack Obama choose an asteroid as the destination for the next great manned mission into space? Learning about these huge space rocks isn't just about science, it's about survival.