My Kingdom for a Master Plan
The episode - "The Dalek Master Plan" is unique in many regards. For one, it's the longest story in Doctor Who history. Set in the year 4000, the Daleks have called together various rulers of different galaxies to form one giant alliance. Of course this is another first, as the Daleks have always worked alone in the past.
Another interesting tidbit, several weeks before this story aired, there was a single cutaway episode of Doctor Who featuring only the Daleks, broadcast as a teaser for this story. As the story unfolds, Mavic Chen - guardian of the Solar System has betrayed Earth and provides the Daleks the ultra-rare element taranium needed to power their Time Destructor.
Arriving on the planet Kembel, the Doctor, Steven & Katarina befriend Bret Vyon and the group learn what the Daleks are up to. Attempting to make it back to Earth, tragedy strikes when Katarina is ejected into space. This is another unique aspect of this story, never before had a companion of the Doctor died while travelling with him.
Bret Vyon was also unique in that he was played by Nicholas Courtney, who later went on to play Brigadier Alistair Gordon Lethbridge-Stewart. A character in Doctor Who history who has worked with almost every Doctor. Bret didn't last long in the story however, as he was killed by Sara Kingdom. Sara is a security agent who was tricked by Mavic Chen into believing Bret (her brother) was a traitor.
The Doctor convinces Sara she was duped and she makes it her personal goal to make sure Mavic Chen and the Daleks are stopped. Along the way, they run into the Meddling Monk. He has fixed his Tardis and is now pursuing the Doctor. Despite all the odds the Doctor is able to thwart the Monk, defeat the Daleks and destroy the Time Destructor. However, Sara Kingdom dies, devastated by the effects of the Time Destructor.
DEPARTURE LOCATION: Above the planet Desperus, 4000 A.D.
REASON FOR LEAVING: Sacrificed herself to save the Doctor, Steven & Bret Vyon.
WHO: Sarah Kingdom
PLANET OF ORIGIN: Earth, 4000 A.D.
REASON FOR JOINING: Sides with the Doctor when she realizes Mavic Chen is the traitor she's looking for.
DEPARTURE LOCATION: Kembel, 4000 A.D.
REASON FOR LEAVING: Sara's body is destroyed by the effects of the time destructor.
NEXT: The Dodo Bird Massacre