Having regenerated for a third time, the Doctor seemed to be in a hurry to leave Earth, but was eventually persuaded to stay and help Brigadier Lethbridge-Stewart deal with Think Tank and their K1 robot. Afterwards, he took Sarah Jane Smith and Harry Sullivan to travel with him in the TARDIS. In the midst of another adventure, a Time Lord appeared to the Doctor and offered him a mission, to intervene in the creation of the Daleks. He was either to prevent it, or to induce the Daleks to develop into less dangerous creatures. Upon his return to Earth and the defeat of the Zygons, Harry decided to remain behind on Earth. The Doctor had began, even more so with this regeneration, to break his ties with Earth. However, despite intending to resign from UNIT, the Doctor never formally left his position as their unpaid scientific adviser.