Sebastian Musgrove was a survey officer stationed on Monitoring Base XL-7, alone except for two drones which he named Drone Gold and Drone Silver. His girlfriend, Tarla McCail, had been killed by Daleks while he was on the base, and he created a hologram of her to keep him company.
Unknown to Sebastian, the continent on which the base was situated was alive and sentient. It was a life form named Telxzana, the last of its kind and dying. It had tried to contact Sebastian, but it was only able to communicate through the base's power systems, causing power outages. Sebastian became aware that something was on the planet with him, and this knowledge was slowly driving him mad.
When the Fifth Doctor and Nyssa visited the planet, they were able to communicate with Telxzana. The Doctor arranged for a transport to take Sebastian off-planet, though he stayed until Telxzana died. (PROSE: Lonely Days)