The Impossible Astronaut (1)

Placing his life entirely in their hands, the Doctor agrees to search for the recipient of the fourth envelope - just who is Canton Everett Delaware the Third? And what is the relevance of their only other clue: 'Space 1969'? Their quest lands them - quite literally - in the Oval Office, where they are enlisted by President Nixon himself to assist enigmatic former-FBI agent Canton, in saving a terrified little girl from a mysterious spaceman.