The TARDIS lands aboard the Sun God, a vast spaceship carrying executives from a powerful 35th Century energy company. Peri is menaced by deadly cat-like robots, while the Doctor discovers that Spalding Revere, the company's founder, has set the ship on course for the heart of a sun. Spalding's last wish is to take his acolytes with him to the afterlife, and unless the Doctor can intervene he and Peri will go with them. Tensions and in-fighting grow among the staff. Peri meets Brian, whose half-man/half-insect body has a terrible significance to Spalding's plan. As the Sun God continues on its final mission, time is running out for them all.