During the Last Great Time War, Unity was a dusty mining planet orbiting a dying star on the fringes of a dying galaxy. Leela lived for years on Unity, with Veega and Rayo, after she was thrown into the Time Vortex by the War Master.
Romana II and Narvin tracked Leela to Unity. Their TARDIS was noticed by Jarred McKenzie, who tried to use it as leverage with the Daleks, bringing them to Unity. (AUDIO: Unity)