Lady Christina de Souza (Michelle Ryan) boards a London bus, escaping from the police after stealing a priceless gold goblet. The Doctor, who is on a quest to trace rhondium particles, sits down beside her. The bus detours through a wormhole to the desert world of San Helios where they encounter a crashed spaceship inhabited by the insectoid Tritovores, a swarm of voracious metallic and airborne stingray-like scavenger creatures.