9. The Curse Of Davros - Wrong Bodies
The Curse of Davros opens with what feels like a classic exciting Who scenario, the Doctor is fleeing from the Daleks and their creator, and his crashed escape vessel is found by Philippa 'Flip' Jackson, who knows the Doctor from a previous adventure. A timey-wimey plot ensues, where the Doctor and Flip must stop a Dalek incursion in the 21st Century, as well as stop a plot at the Battle of Waterloo. However not all is as it seems...
One of the story's cliffhangers reveals that the Doctor is in fact not the Doctor, but Davros in his body. The Dalek's plot at the Battle of Waterloo was to have Napoleon win, thus changing history. To foil this, and also reform the Daleks, the Doctor uses Davros' mind exchange machine. However, not all goes to plan when Davros escapes and tricks Flip into assisting him to stop 'Davros' and the Daleks.
This moment really shocks listeners as there is no prompt on your first listen that not all is as it seems. Perhaps this story works best on audio as all you have is the seminal voice performances of Colin Baker and Terry Molloy pulling off both roles, but no visual cues to make you suspect anything.