64. The Time Monster

The season 9 finale has everything in it- the Doctor vs the Master, Greek gods, the sinking of Atlantis (again), UNIT, and a swank new TARDIS console room (that lasts only one story)! Clocking in his second appearance of the season, the Master returns to harness the power of Greek Titan Kronos for universal domination. He once again uses a flimsy fake name and is quickly discovered. Amazingly the Doctor is able to pilot the TARDIS back to ancient Atlantis to pursue the Master. The Master seduces the Queen of Atlantis, kills off the King, and unleashes Kronos who wrecks Atlantis. For a 6 episode story, it's actually a pretty well paced story and doesn't feel drawn out. And the final moments of the story with a naked Sergeant Benton and the laughing UNIT family is a precious moment as the Doctor's time in UNIT's employ begins to wind down.

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