58. Colony in Space

After a season and a half, the Doctor finally leaves Earth for an alien planet. And we begin the "Time Lords send the Doctor on a mission" recurring theme. Jo gets her first ride in the TARDIS. The Master shows up, of course, by episode 4. Good time for all!

The most exciting thing about the story is seeing the interior of the Master's TARDIS for the first time. We get to see the Time Lords on their home planet discussing the dispatch of the Doctor after the Master steals the Doomsday Weapon file.

This story could probably have been 4 episodes instead of 6 but otherwise a fun romp in a quarry, I mean alien planet.

http://en.wikipedia.org/wiki/Colony_in_Space