This is such a strange segment of the story. The Cybermen aren’t in it, for starters. They only appear in the reprise of the previous episode. Frazer Hines and Edward Burnham are only in a single scene, in the set used at length in part six but only here for about one minute. I think this scene was taped along with part six so that the actors could still get a week off.
The central bit of drama is the Doctor trying to convince Vaughn that he’s in way over his head, and it’s Zoe who ends up proving him right. Over at a nearby air base, she immediately recalculates some surface-to-air missile coordinates and UNIT and the RAF shoot down the Cybermen’s incoming transport ships. So that “science machine” in Vaughn’s closet – actually called the “Cyber-Director” – ends the episode ending the alliance with Vaughn and announcing they’re going to destroy all life on earth.
Zoe is certainly amazing, but her timing might not have been perfect this time.
So it doesn’t seem like a lot actually happens in this episode, and some of it was certainly over our five year-old’s head. But there’s an ongoing, oppressive sense of worry and danger. When the Doctor goes down into the sewers to rush to Vaughn’s headquarters, our son realized that he didn’t have any of his security blankets handy – it almost hit eighty degrees today, in February!, so he wasn’t wrapped in any – and so he started chewing on his mother’s thumb because he was so afraid for the Doctor.