Doctor Who: 10 Unresolved Storylines & Mysteries That Need To Be Explored

The Doctor finally gets round to investigating the "Egyptian goddess loose on the Orient Express. In space" mystery. The Egyptian goddess in question is the Foretold, a supernatural being in mummified form, who kills anyone who can see it after 66 seconds have passed precisely.

However, the Foretold is the misunderstood villain of the piece, despite taking the lives of several people in a pretty harrowing manner. The Doctor works out, naturally, that all the alien experts aboard were deliberately brought there by "someone of immense power and influence", to discover the Foretold's raison d'être.

Enter good old Gentleman Gus, brilliantly voiced by John Sessions.

The Doctor does his thing and figures it all out, naturally, and saves Clara, Perkins and the remaining passengers before Gus can dispose of them too. He tries to find out who or what is behind the whole scheme by trying to hack the sinister supercomputer, but to no avail as Gus covers his tracks and blows up the train.

Writer Jamie Mathieson originally planned on incorporating Gus in series 10's Oxygen. But with that idea scrapped, we're left to speculate. As Gus would say: Isn't this exciting?

With a penchant for casual killing, the short odds are obviously on the Master/Missy. Plus, she was a dab hand at manipulating events by dropping the Doctor a line on the TARDIS telephone. Interestingly, if this fan theory is anything to go by, perhaps another Time Lord is playfully hinting they've rediscovered their true renegade self - it's about time we had another flavour alongside M&M.