Season thirteen consisted of five four-parters and one six-parter, establishing a pattern that would be repeated for the next four years (although season seventeen would lose its six-parter due to industrial action). This season was wholly the responsibility of Philip Hinchcliffe and Robert Holmes, who were thus able to ensure that it fully reflected their shared vision of the series. The horror content of the stories was even higher than before, and the gradual phasing out of UNIT that had been begun in the third Doctor's era was finally completed.
Harry Sullivan - a character originally conceived as someone who could take care of the action scenes if the fourth Doctor happened to be played by an older actor - was also now considered surplus to requirements and written out. The resultant season was not only one of significant change, however, but also one considered by many to be amongst the very best in the series' history.
The departure of Harry as a regular at the end of Terror of the Zygons left the Doctor with only one companion, as had been the case during the Jon Pertwee era. The difference this time was that UNIT were also written out during the course of the season.