Doctor Who: 10 Storylines That Will Never Be Revisited

10. The Identity Of Gus

The main evil force in Mummy on the Orient Express was the bandaged zombie of the title, which gave its targets 66 seconds to live before killing them without mercy. However, the episode also teased a bigger, behind-the-scenes bad guy called Gus, but this intriguing setup ultimately went nowhere.

The episode established that Gus - basically an evil supercomputer - was responsible for unleashing the mummy on the unfortunate passengers of the train. This was done so that he could analyse the creature, capture it, and use its abilities for untold nefarious deeds. But when the Doctor defeats the mummy at the end of the story, Gus blows up the train and vanishes, never to be heard from again.

This left the identity and origin of Gus a total mystery: who was behind the computer? Considering that it's been years since we last saw the character and he was hardly an immensely popular breakout villain to begin with, it doesn't look like we'll ever get an answer to that question.

We did come close, though. Gus was almost included in Oxygen, an episode that aired two seasons later, but this idea was scrapped early on.