'The TARDIS lands on a spaceship that is standing on the surface of a pleasantly fertile planet. The space ship has a number of bulkhead doors, all sealed, and no evidence of a crew. Through a window in one door they see a young woman on a bed, apparently in a coma. The Doctor says that a decontaminant agent has been used in the air supply, suggesting that the woman and/or the crew have suffered an infection. While the Doctor makes his way laboriously through the doors Sarah steps out of the ship onto the grass outside, noting that the ship landed safely and didn't crash. The Doctor finally encounters a soldier, Lieutenant Hrinth, who is wearing a containment suit. He tells the Doctor that the ship was carrying Princess Catra from Arrada to her wedding on Voita. Her marriage to Prince Strobel will end a three year war between the planets. However, a drive failure meant the ship had to put down on this planet. Hrinth was part of a recovery mission but when they tracked down the princess she was in a coma and the crew of her ship had vanished or died. The recovery team fell victim to a suspected virus which caused them to lose the ability to read, as well as a susceptibility to hearing voices. The other rescuers died when their cook lost the ability to read and poisoned them all. Sarah re-enters the ship but she has now lost the ability to read. A group of humanoids arrive at the ship; they have mouths but no other facial features. The Doctor uses telepathy to communicate with the aliens and discovers that the virus was actually a pheromone which they use to communicate. For the Arradans, unfortunately, this meant that the communication centres in their brains were suppressed, which is why they could no longer read. The Doctor injects Sarah and Hrinth with a drug to combat the pheromone but tells the Lieutenant that Catra does not want the drug, choosing instead to remain on the planet. Hrinth says that if he does not return with the princess he will be executed by the Royal Family. The Doctor offers to take him, and his family, to a safer world, but Hrinth uses the injection on the princess. It leaves her as a mobile but unthinking shell of her former self. Disgusted, the Doctor prepares to leave but uses the TARDIS's telepathy to let Catra say farewell to her friends on the planet.'
[The Doctor is the Third Doctor; his companion is Sarah Jane Smith.]
Source: drwhoguide.com (http://www.drwhoguide.com/whotrip28.htm). Sighted 27/5/11