It’s difficult to say whether the runtime exceeding three hours was truly necessary, even considering the recap segments, but despite that, it ranks among the stronger and more unsettling adventures featuring the Doctor (Patrick Troughton), Jamie (Frazer Hines), and Zoe (Wendy Padbury) – I had completely forgotten about her character! The story begins with their arrival on Earth to meet up with the recently promoted Brigadier (Nicholas Courtney), coinciding with a series of disappearances among leading scientists. With UNIT assigned to look into it, naturally the Doctor is eager to assist in discovering where they’ve gone and what role the intimidating industrialist Vaughn (Kevin Stoner) might play. Soon, the group finds themselves caught up in a puzzle that can only indicate an alien invasion – but by whom, when, and where? The picture becomes more defined roughly an hour before the end; truthfully, much of the first two hours consist of a leisurely introduction that could easily have been shortened to about half its length. However, once the familiar antagonists appear, the program truly takes off. Not hesitant to take advantage of the BBC’s visual and sound effects capabilities, the production team creates some of the finest 1960s science fiction danger you're likely to encounter on film. Troughton is at his best with his playful delivery, although I did find Padbury somewhat irritating; nevertheless, there’s a consensus among those involved that this represents another reason why it was essential television viewing.

Source on TMDB