Sebastiene lived on and was master of the sentient Planet 1. He lived in the Chateau, a sprawling, illogically designed eighteenth century Earth style palace of his own creation. He was fond of things from the 18th century, habitually dressed in 18th century clothesm and made all his robot servants grow moustaches in the style of the era.

He claimed to be a human student who came from Earth in 1973 when attempting to gain Donna Noble's trust and trick her into assisting him, but the Doctor was not able to confirm this.

He was an expert sportsman who hunted and collected rare creatures, including Mmtefl Beasts, Zargregs, Moogs, Daleks, a Raston Warrior Robot and even an Eternal, all of which he killed and placed on display in a large trophy room. His ultimate trophy was to have been the Tenth Doctor, the last Time Lord, and he forcibly recruited the Endangered Dangerous Species Society to hunt the Doctor for his amusement.

His plan failed when the Doctor switched places with Baris, gained access to Planet 1's computer systems and, with the assistance of Planet 1 itself, was eventually able to escape. Planet 1 decided it didn't need him anymore, and he promptly with the help of the Doctor, Donna and Baris, destroyed a large part of its northern continent using explosive decoy robots[statement unclear]. He tried to hold the Doctor hostage on the condition that he take him from Planet 1, but the Doctor refused. Sebastiene was then attacked by the last remaining hunter, the Carpalian Witch, and was last seen fighting it off with his sword alongside Baris. (PROSE: The Doctor Trap)