Valtar met the Doctor when he visited Vaga and helped them develop several technologies, including an inter-dimensional craft. Valtar was one of the few survivors when the Pyrons destroyed one of their suns, having left in the craft. He returned to Vaga but delayed landing as the Doctor's TARDIS was there. Had both landed, the planet would have been destroyed.
They finally landed on the planet with the Key of Vaga and used a beta-force field to prematurely detonate a Pyron space-shark's warheads, destroying it. Valtar used a hologram of himself to greet the Doctor and explain the situation before telling him they were leaving, never to return to the planet. (PROSE: The Key of Vaga)