Having re-watched this recently after a couple of years, it is a cracking way to introduce Turlough, bring back the Brigadier and encompass all the plots/ subplots in a good story.
Although the story has some weaknesses including the bizarre costumes for Mawdryn and the gang plus all the time line nonsense, which mucks up continuity, the pluses outweigh the negatives.
Turlough is extremely well acted by Mark Strickson and is a shame that the potential in this story isnt really enlarged upon until his final story. Peter Davison too excels as the Doctor with breathless enthusiasm especially in parts three and four where he is willing to give up his remaining lives to save his friends.
Although Nyssa and Tegan are bound by the TARDIS, they are effective in their roles and helpful as a plot device in the Doctors decision to help Mawdryn.
And then there is Nicholas Courtney who is brilliant as the Brigadier and nice to see him mellowing as a schoolteacher and not as the baffoon he did during the later Pertwee stories.
Peter Grimwade does write a very complicated story, which is very different to most of the stories surrounding it. It is a million times better than his last story (Time-Flight). However you do just think the Black Guardian is pure evil and can kill and destroy if he wants. So why doesnt he just kill the Doctor himself?
This is on the other hand a minor detail and does not really deter from a really good story, which is sometimes a forgotten gem in Davisons era. 8/10