Vagan
The Vagans were a peaceful and highly developed race from the planet Vaga. The Doctor helped the Vagans build an inter-dimensional craft and the Key of Vaga to help them return to the planet at will.
The neighbouring Pyrons destroyed the largest of Vaga's suns with a plinium missile. The Vagans were wiped out within one Vagan-year, save a few who left in the inter-dimensional craft.
The Fifth Doctor and Adric tried to stop the Vagan craft returning to Vaga as its presence there alongside the TARDIS would destroy the planet. The Vagan were already aware of this and arrived some time after. Using a beta-force field supplied by the Doctor when he helped build it, the Vagan craft prematurely detonated the Pyron space-shark's warheads, destroying it. Although they were opposed to violence, they though it fitting that the Pyrons were destroyed by their own weapons of destruction.
Valtar, a former friend of the Doctor's, used a hologram to talk to him and Adric, explaining how they were aware of their predicament and so had avoided landing. Following this, the Vagans left their planet, supposedly forever. (DWA: The Key of Vaga)
