Considering that Doctor Who is almost sixty years old, it's really not surprising that a vast amount of story has been left by the wayside throughout its run.
After all, writers, producers and showrunners eventually move on, actors leave - often taking their associated plot points with them - and audience reaction can dictate what does and doesn't appear in the show, creating even more abandoned narrative threads.
And when you factor in the standalone nature of most episodes - as well as the time travel element, which can easily overwrite and change certain things - it's pretty much inevitable that some stories will just become lost within the huge, all-consuming time vortex of wibbly-wobbly doom... otherwise known as Doctor Who history.
Bearing that in mind then, here are some storyline setups, character arcs and plot ideas that we still haven't revisited - and probably never will. While some of these may have resurfaced in comic books, novels and audio dramas, the main TV show is the central pillar of Doctor Who canon, and in this area, we've been left in the dark.
10. The Truth Behind The Midnight Entity
One of the most chilling episodes in Doctor Who history, Series 4's Midnight pits the Doctor against a terrifying, powerful - and most importantly - unseen enemy.
The Midnight entity approaches the Doctor (and a group of citizens) while he's trapped inside a broken-down tour bus, and is somehow able to enter said bus without any of its passengers noticing, and without leaving a single trace of damage. It then stealthily possesses people, never showing its true form.
While the entity is seemingly vanquished at the end of the episode - and even that isn't a sure thing - it remains one of the only monsters in the universe that the Doctor knows nothing about, and we haven't seen it in another episode since Series 4 aired.
So what exactly is the Midnight entity? What does it look like? Are there more?
While there have been plenty of theories suggested by fans, we don't know the answers to any of these questions, which is unusual because Doctor Who, normally, does at least do some explaining about its bad guys.
That being said, the episode was clearly deliberately designed to reveal as little information as possible, and the lack of details about the entity only enhances the suspense. So it wouldn't be a total tragedy if it went unexplained permanently.