Deneb 7 was an uninhabited planet. A delegation of robots arrived in orbit around the planet and sent Pennant Bearer to the surface to negotiate with the inhabitants for colonisation rights. However, during the trip down, the Z-Bomb he was carrying exploded, destroying him and Deneb 7. The other robots remained inactive in their spacecraft, waiting for Pennant Bearer to return.
Four hundred million years later, the Fourth Doctor and K9 arrived on the robots' ship, having mistaken it for a moon. While the Doctor slept, K9 went exploring and accidentally revived the robots, who mistook him for Pennant Bearer. He brought them to the TARDIS, where they learned of the destruction of the planet. They returned to their ship to wait, inactive, for a new planet to coalesce from the dust cloud that had been Deneb 7, a process which would take four billion years. (PROSE: Touchdown on Deneb-7)