The one where Ben and Polly go back to the start...
In keeping with the sedate pace of the preceding five episodes, the "finale" of The Faceless Ones jogs to the finish line rather than races. Writers David Ellis and Malcolm Hulke have an essentially interesting story on their hands, but it's been plotted and executed somewhat clumsily, making anything which could have been exciting mildly underwhelming. Hulke would go on to pen some of Doctor Who's most intelligently written stories in the Pertwee era (not always the most dynamic, though), while this was Ellis's only successful contribution to the show (he'd submitted various storylines to the Doctor Who office previously, including one called The People Who Couldn't Remember). Sadly, both writers died just 12 months and one week apart, in June 1978 and July 1979.
Their combined legacy has to be regarded as one of the weakest stories in Season 4, but that's not to say there aren't kernels of strong ideas. It's just executed in such a disappointingly uncertain way. Credulity is stretched regularly. For instance, could the Commandant really put a very sudden halt to all outgoing flights from Gatwick just to rally the airport staff to search for 25 hidden bodies? The repercussions for international air traffic would be devastating (which is probably why Brussels gives him a call at the end of the episode!).