In some ways, Jon Pertwee's Doctor Who stories are very well served by being presented in the audio format as some of his shows have not aged particularly well visually. They are from an era when television had a demonstrative pace of storytelling which was not frenetic or overtly visual and so the story unfolds very well in sound only, giving you the opportunity to hear the excellent scripts and performances without being put off by a slightly dodgy monster or effect.
As such these releases provide an excellent introduction to Jon Pertwee's slightly eccentric father figure style of Doctor, and as you go along with the story you warm to him, whereas on video he can seem a little less "matey" than some of the more recent performers of the part. This story comes towards the end of his time as the Doctor, and yet his performance is still as enthusiastic and warm as ever. Jon Pertwee's reputation as the Doctor seems to have diminished over the years, and this is unfair as his stories are never less than entertaining, and for a generation of people, Jon Pertwee quite simply IS the Doctor!
The prequel to this, "The Curse of Peladon" is also available in this format, and both work to create a very convincing alien society which seems to have a believable history. They also include the last appearances of one of the major monster creations of the old series - The Ice Warriors - and expand on their culture and society too. You might not need to know "The Curse of Peladon" in order to enjoy this story, but it's certainly worth a look.
Elisabeth Sladen is an lovely narrator (I could listen to her all day) and the bonus interview with her is up to her usual honest and astute standard.
UPDATE JANUARY 2010 - These episodes have now been released on DVD in a set alongside THE CURSE OF PELADON.
I agree with the previous reviewer but would give this 5 stars not 3. This is an excellent story and well executed. It ports well to audio because your imagination takes over and compensates for the missing visuals. I listened to this and the 'Curse Of Peladon' BBC Audio soundtrack version together. Both stories complement each other. Highly recommended.