This is the best and final episode of Season 7, starring Jon Pertwee in the role of the Doctor, with Caroline John as Liz Shaw, the Doctor's Assistant as well as the UNIT folks. Like many stories from this era, there is a not unnoticable "ecological" vibe throughout the story.
The only downside to this story is that clocking in at 7 episodes, it is at least one episode too long. On the plus side, we get to visit an alternate universe, where we get to see the evil versions of Liz, the Brigadier and Benton amongst others. I recommend to buy this one.