The last episode to feature the companions Ben Jackson and Polly Wright.
Ben and Polly only appear in their goodbye scene, which was pre-filmed.
This episode was wiped by the BBC and no copy of it is known to exist.
Ben's remark that it is the same day, 20th July 1966, that he and Polly left Earth with the Doctor means that the action at Gatwick happened simultaneously with the attempted coup d'etat in the centre of London by super-computer WOTAN, as described in an earlier story The War Machines. Apparently news of this did not reach Gatwick, which is only half an hour from London. It also means that, for a period of time, two versions of Ben and Polly existed simultaneously.
This episode takes place in London on July 20, 1966.