The one where the real Queen Xanxia reveals herself...
It wasn't the Doctor who fell 1,000ft to his doom last week, it was actually an image of the Doctor projected using a gadget he found in Xanxia's throne room. But if the Doctor we saw giving himself up on the Bridge last week was a projection, how did he manage to hand the dead Polyphase Avatron to the Captain? How could a non-corporeal projection carry a solid metal object? I won't dwell on that too much, I'll let you readers do that...
The discovery of the image projector lets Queen Xanxia's cat out of the bag when the Doctor reveals that the Captain's severe young Nurse is actually the newly regenerated form of the tyrannical ruler of Zanak. The decaying shell in the throne room is merely a husk, and the real Xanxia has been busy projecting a new image of herself - a younger, foxier, sexier image - using the power generated by Zanak's planet-crunching shenanigans. The imagination in this story is astounding.