S1 E2 - End Of The World
From the wiki:
"The End of the World" is the second episode of the first series of the British science-fiction television programme Doctor Who. Written by show runner Russell T Davies and directed by Euros Lyn, the episode was first broadcast on 2 April 2005.
In the episode, Time Lord the Doctor (Christopher Eccleston) takes his new companion Rose Tyler (Billie Piper) on her first trip through time and space in the TARDIS to the year five billion where many rich alien delegates have gathered on a space station called Platform One to watch the Sun expand and destroy the Earth, but one of these guests is plotting to profit from the event by killing them all.
The episode marked the first appearance of Cassandra and the Face of Boe. The episode was seen by approximately 7.97 million viewers in the United Kingdom.