The third, fourth and fifth stories of the twentieth season were conceived by John Nathan-Turner and Eric Saward as a trilogy reintroducing the Black Guardian in the BBC sci-fi drama.
Mawdryn Undead The Black Guardian recruits a young man named Turlough (Mark Strickson) to assassinate the Doctor (Peter Davison). Although outwardly an ordinary pupil at a boys' private boarding school, Turlough is in fact an alien who believes that the Guardian will return him home if he succeeds.
Terminus The Tardis attaches itself to a space liner after Turlough (Mark Strickson), still under the Black Guardian's influence, damages its controls. The Doctor (Peter Davison) and Nyssa (Sarah Sutton) meet two space pirates, Kari (Liza Goddard) and Olvir (Dominic Guard), who have come on board the liner in search of plunder, while Tegan (Janet Fielding) and Turlough get lost in the infrastructure.
Enlightenment The White Guardian warns of impending danger and directs the Tardis to what appears to be an Edwardian sailing yacht, the SS Shadow, but is actually one of a number of spaceships taking part in a race through the solar system, the prize being Enlightenment.