I loved this story. Really loved it. I was so hyped after this story that we went out to dinner after watching it and I was sitting in the booth dancing to the restaurant's music. The story is six episodes long. Normally, for most classic stories, we watch a couple of episodes a night. We were so hooked on this one that we did it all in one sitting and when it ended, we went, "That was six episodes??"
Why was it so engaging? First, a plot that kept going, throwing curveballs in the path of the heroes, who consisted of not only the Doctor and Jo, but the Brigadier, Sgt. Benton, Capt. Yates, and the main guest character Cliff Jones and his staff. Even the second episode, which consisted primarily of Jo trying to escape a dark mine with a miner and complaining about how exhausted she was, never dragged. Second, interesting guest characters and a fantastic development of the relationship between Jo and Cliff. Also, the main human villain was fascinating, right up to the end. (The other main villain, which I won't name so as not to spoil a forty-five-year-old story, was not fascinating, though it did add a nice surreal touch to the end.) Third, a number of well-placed humorous scenes, including the Doctor disguising himself as a cleaning lady and trying to maintain his dignity in front of Capt. Yates and the Brigadier getting accidentally hypnotized.
Of course, this was Jo Grant's last episode, and she got a fantastic send-off. She meets environmental activist Cliff Jones, and they grow close over their shared interests and purpose. By the end of the episode, she's found her calling and chooses to leave the Doctor to accompany Cliff on his search up the Amazon for the ultimate clean food. Against his personal wishes, the Doctor graciously wishes her well and, hating goodbyes as much as always, slips out as UNIT and Cliff's staff celebrate the couple's engagement.
One thing I noticed was that this story often threw in things I didn't expect. For example, once the Doctor de-hypnotized Yates, he sent him back to the villain Stevens to pretend he was still hypnotized to get information. I expected Yates to report to Stevens that the Doctor was dead and that Stevens would then enact his plan assuming no further interference. Instead, Stevens immediately suspected Yates, uncovering the plot and then imprisoning him. Another (complex) example was the Brigadier's use of Yates as "the man from the Ministry", and then Yates refusing to let the Doctor do what he wanted (to maintain the disguise and make Stevens trust him). These long stories simply have the time to drive the story off into new territory and allow things to slowly get back on track, and this one did it well.
I was especially pleased with Mike Yates in this story, as his role in the story was unusual, pretending to be a government official on the villain's side, and he played his part well, feeding the Doctor and the Brigadier information from right under Stevens' nose. Benton was his usual sergeant-y self, and I wonder if John Levene ever wished he'd got to play something different. (Not that Benton isn't awesome. He is always loyal to the Doctor and the Brigadier, and sharp as a tack.)
Now, four hours later, I'm still yammering about this story. I have a feeling I'll be dreaming about maggots tonight. Um. Yuck.