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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.
Mawdryn Undead:
The Black Guardian recruits a young man named Turlough to assassinate the Doctor. 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, still under the Black Guardian`s influence, damages its controls. The Doctor and Nyssa meet two space pirates, Kari and Olvir, who have come on board the liner in search of plunder, while Tegan 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.
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