Season 13
It was during this season that the Doctor cut ties to UNIT (after Terror of the Zygons, the Brigadier would not appear again until Mawdryn Undead in 1983, and The Seeds of Doom marked the last occasion to date that the Doctor was explicitly said to be working for UNIT - as opposed to working with them as occasions warranted). Several of the serials in this season, particularly Pyramids of Mars and The Brain of Morbius, are among the best-known of the 1970s-era storylines. Though it was aired as a part of Season 13, Terror of the Zygons was actually filmed as part of the previous season; this holdover allowed the series to switch back to an autumn season start.
Episodes
1 Terror of the Zygons (1) Aug 30, 1975 
2 Terror of the Zygons (2) Sep 6, 1975 
3 Terror of the Zygons (3) Sep 13, 1975 
4 Terror of the Zygons (4) Sep 20, 1975 
5 Planet of Evil (1) Sep 27, 1975 
6 Planet of Evil (2) Oct 4, 1975 
7 Planet of Evil (3) Oct 11, 1975 
8 Planet of Evil (4) Oct 18, 1975 
9 Pyramids of Mars (1) Oct 25, 1975 
10 Pyramids of Mars (2) Nov 1, 1975 
11 Pyramids of Mars (3) Nov 8, 1975 
12 Pyramids of Mars (4) Nov 15, 1975 
13 The Android Invasion (1) Nov 22, 1975 
14 The Android Invasion (2) Nov 29, 1975 
15 The Android Invasion (3) Dec 6, 1975 
16 The Android Invasion (4) Dec 13, 1975 
17 The Brain of Morbius (1) Jan 3, 1976 
18 The Brain of Morbius (2) Jan 10, 1976 
19 The Brain of Morbius (3) Jan 17, 1976 
20 The Brain of Morbius (4) Jan 24, 1976 
21 The Seeds of Doom (1) Jan 31, 1976 
22 The Seeds of Doom (2) Feb 7, 1976 
23 The Seeds of Doom (3) Feb 14, 1976 
24 The Seeds of Doom (4) Feb 21, 1976 
25 The Seeds of Doom (5) Feb 28, 1976 
26 The Seeds of Doom (6) Mar 6, 1976