I feel this story is very underrated. Yes the effects are poor and it has some other faults but it also has a lot of strengths.
This is the first story for Peter Davison as the new Doctor (the 5th) and he is brilliant despite being hampered by the story giving him a 'difficult regeneration' that incapacitates him for much of the first few episodes. In every moment that he is speaking and involved Davison shines. There are echoes of all 4 earlier incarnations as his regeneration causes his mind to wander to the past and Davison does great impressions of the first and second Doctors. His mental and physical struggles are very well acted and it is pretty interesting to see a regeneration not going smoothly in this way. The 3rd Doctor had some similar issues so this was a logical extension of that idea. As the story progresses Davison quickly establishes his Doctor as an energetic, positive, caring and heroic personality and shows signs of being a motivating 'team Captain'. I feel this characterisation seems to have been echoed closely by the characterisation of the 13th Doctor many years later. It was a thankless task to follow the legendary Tom Baker but Davison does a fabulous job managing to be a very different but very enjoyable and endearing version of the character.
The effects are very weak and that is a negative aspect as is some time wasting in episode 2 when Nyssa and Tegan carry a large box around the woods which is boring and pointless. That makes that episode the low point but even in episode 2 there are lots of plus points. Right throughout this story all 3 assistants are at their best with all 3 serving a purpose and delivering some intelligent dialogue. Episode 2 has some interesting scenes with Adric captured in 'the web' by the Master utilising his mathematical skills to create a trap for the Doctor. Adric is quite impressive in more ways than one! Adric suffers in the web (although Matthew Waterhouse seems to enjoy it!) and despairs at being used by the Master. Acting from all 3 companions is nothing special but is OK and they are given some good material apart from the journey through the woods. Nyssa uses her brains to good effect and Tegan is feisty as she tries to help. Meanwhile Anthony Ainley shows his acting abilities as he shows a totally different side whilst in disguise proving his cackling villain portrayal of the Master is only one side of him.
The plot is hard to follow or understand but it works somehow and the scientific depths explored give an heir of intelligence and cleverness in the script. I am thrilled that there is no dumbing down and that intellectual ideas are attempted. I find the plot interesting and the dialogue delightfully ambitious.
Michael Sheard makes another guest appearance in Doctor Who and, as always, is superb. He imbues his relatively small role with a lot of depth. Other guest actors are solid and the script is strong. With better effects and production this could have been even better but I still think it is a very good adventure that has some ambition and a lot of qualities.
My ratings: Episode 1 - 9/10, Episode 2 - 8.5/10, Episode 3 - 9.5/10, Episode 4 - 9/10. Overall - 9/10.