Lady Christina de Souza boards the Number 200 red double-decker bus in London, hoping to make a quick escape from the police pursuing her after she has stolen a priceless gold goblet from the International Gallery. However, what she is not expecting is for the Doctor to sit down beside her, trying to trace Rhondium Particles. Nor does she anticipate that a wormhole will open up in front of the bus, causing her, the Doctor, the other passengers and the driver to be transported far across Space, to the desert world of San Helios. There, the Doctor and Christina encounter a crashed spaceship, inhabited by the insectoid Tritovores, as well as a swarm of voracious metallic and airborne stingray-like scavenger creatures, ready to use the wormhole to reach Earth, where they will feed until they reduce the planet to a desert, just as they did on San Helios- Can the Doctor again save the world?