The Fourth Doctor was the fourth incarnation of the Time Lord known as the Doctor. He left all ties of his previous incarnation's exile behind on Earth, setting off to travel the cosmos. He initially travelled with Sarah Jane Smith and Harry Sullivan, but Harry chose to remain on Earth when the opportunity arose. The Doctor and Sarah Jame continued on their adventures until the Doctor was summoned to Gallifrey, necessitating that he leave Sarah Jane behind as humans were not allowed on Gallifrey at that time.