Now i'll start off by saying i'm not the worlds biggest fan of Pertwee's tenure as the Doctor. However this is a solid little story.
What lifts it above a lot of Pertwee's stories is the script which is excellently written by Robert Holmes.
Also each actor plays their character to perfection, usually with 70's Who you get some proper stinky acting but here everyone gives their best. Katy Manning is at her best as Jo
The Drashigs hmmmm i don't see why they get the ridicule of the fandom. The efects are convincing, doubly so when you consider that it was made in 1972 with less money than it costs to buy a mars bar.
The design is colouful and eye cathing. All in all a good little story.
One last plus id that it finally ends The Doctors exile on Earth. Although why they bothered is beyond me as the new series has the Tenth Doctor on Earth all the time even though he isn't exiled!!!