Season 20
Structurally, the season retread season 18's formula of including a trilogy of stories bound together by an overarching theme. In this instance, the tripartite tale involved Turlough's attempts to kill the Doctor at the behest of the Black Guardian.
Episodes
1 Arc of Infinity (1) Jan 3, 1983 
2 Arc of Infinity (2) Jan 5, 1983 
3 Arc of Infinity (3) Jan 11, 1983 
4 Arc of Infinity (4) Jan 12, 1983 
5 Snakedance (1) Jan 18, 1983 
6 Snakedance (2) Jan 19, 1983 
7 Snakedance (3) Jan 25, 1983 
8 Snakedance (4) Jan 26, 1983 
9 Mawdryn Undead (1) Feb 1, 1983 
10 Mawdryn Undead (2) Feb 2, 1983 
11 Mawdryn Undead (3) Feb 8, 1983 
12 Mawdryn Undead (4) Feb 9, 1983 
13 Terminus (1) Feb 15, 1983 
14 Terminus (2) Feb 16, 1983 
15 Terminus (3) Feb 22, 1983 
16 Terminus (4) Feb 23, 1983 
17 Enlightenment (1) Mar 1, 1983 
18 Enlightenment (2) Mar 2, 1983 
19 Enlightenment (3) Mar 8, 1983 
20 Enlightenment (4) Mar 9, 1983 
21 The King's Demons (1) Mar 15, 1983 
22 The King's Demons (2) Mar 16, 1983