43. The Wheel in Space

The Cybermen, again?! Hello Zoe! As the season began, so ends the season with our friendly neighborhood Cybermen paying a visit. This time they're just floating around in space. We are also introduced to new companion Zoe, a smart-as-a-whip computer genius, who tries to out-smart the Doctor at every turn.

Only two episodes of this story exist out of its original six. This really caps off a season that almost entirely no longer exists. From the film snippets and random episodes that do still exist, the tragedy of their destruction continues to compound itself.

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

Next thrilling episode- season 6 begins with The Dominators!