This is the oldest surviving episode to feature the Doctor's face in the opening title credits.
The last ever story to feature Ben And Polly.
This episode takes place in London on July 20, 1966.
Malcolm Hulke and David Ellis' original pitch was called The Big Store, which involved aliens who had taken over a London department store and were using it as a front for their invasion of Earth. One group of aliens was a faceless breed, artificially given human features in order to replace people kidnapped in the store. It was decided that an airport would be a better setting.
The working title for this story was The Chameleons.