The Black Guardian Trilogy took place in Season 20. The episodes were Mawdryn Undead, Terminus and Enlightenment.
The trilogy starts in Mawdryn Undead. After losing the Key To Time, the Black Guardian asks Vislor Turlough, a young Trion exiled on Earth to kill the Doctor in order to return to his on planet. Surprisingly, The Doctor’s old friend the Brigadier is as the same school Turlough attends as a Maths teacher after retiring from UNIT. He ends up on the TARDIS following the Black Guardian’s instruction which lead to more incidents and the Doctor meeting 2 versions of the Brigadier. Later, Nyssa and Tegan end up in danger after touching Mawdryn and the Doctor is forced to lose his remains regenerations to save His friends but luckily the two Brigadiers touch (ZAP) and the Mawdryn and mutants die. Turlough luckily escapes being abandoned on the ship and aks to join the team which the Doctor allows.
Turlough’s addition to the team makes Tegan Jovanka suspicious of his behaviour and they are paired up for the majority of Terminus when he messes the TARDIS controls where Nyssa is forced to leave through a unknown door that leads to another spaceship. Nyssa ends up with the Lazars disease but is later cured and ends up stating behind to help the Vanir find a cure whilst Turlough is given on final chance by the Black Guardian.
Later in their timeline, the TARDIS loses power as the White Guardian attempts to convey a message and they end up on the spaceship Shadow captained by Striker which is competing in a race to win Enlightenment against ships like the Buccaneer which is captained by Wrack, Davey’s Ship, Critos and other ships. Wrack is working with the Black Guardian to win the race. They are successful with destroying Critos and Davey’s Ship. They later attempt to destroy the Shadow by using Tegan’s tiara when they visit the ship to collect Turlough who fell overboard after trying to fight the Black Guardian’s influence. They were unsuccessful and after attempting to ambush the Doctor, Turlough saves him and Wrack and first mate Mansell are thrown overboard. The Bucaneer wins the race despite no crew and the White and Black Guardian’s wait to give them the prize of Enlightenment to the winner when the Doctor, Turlough and Tegan arrive and Striker and Marinner disappear into the Void. White Guardian offers a share of enlightenment to Turlough who ends his contract with the Black Guardian by throwing the enlightenment at him and he burns as dark can exist with out light. White Guardian leaves whilst the Doctor accepts Turlough’s decision to go home to Trion.
I love all 3 episodes as I can enjoy them all. The Black Guardian is used much more than he was in The Key To Time as he was only in The Armageddon Factor. Turlough is a great character as we see him struggle to side with the Doctor or the Black Guardian. Nyssa has a great exit as she feels she can help change Terminus and improve the treatment.