For decades, serious astronomers and paleontologists considered the possibility that our Sun has a dark, unseen companion star that periodically hurls comets at Earth and resets the biological clock of the planet. This episode traces the birth, life, and death of the Nemesis hypothesis, starting not with a telescope but with marine fossils analyzed at the University of Chicago in 1984.

We look at how David Raup and Jack Sepkoski found an apparent 26-million-year rhythm in mass extinctions, why two teams in Nature proposed a red or brown dwarf on a highly elliptical orbit that disturbs the Oort cloud, and how a companion star could hide by sharing the Sun's motion through the galaxy. We examine the strange orbit of Sedna as circumstantial evidence, the infrared surveys from IRAS to 2MASS to WISE that found nothing, and the two blows that end ...