Doctor Who actor Peter Purves is in Preston this morning as students at the University of Central Lancashire start a project to film a lost episode. The students will spend the next three days filming Mission to the Unknown in order to bring the story back to life. Here are six facts about the missing Doctor Who episode known as Mission to the Unknown or Dalek Cutaway.

1. It is short but sweet – It is the only single, twenty-five minute story in the programme’s history.

2. The original footage did not survive – The episode was part of the third season of Doctor Who which was first broadcast on October 9, 1965. Only audio recordings of the episode exist. The videotaped episode was wiped in July 1974.

3. It is the only story not to feature the regular cast including the Doctor himself who was played at the time by William Hartnell. It doesn’t feature the TARDIS at all and the Doctor’s companions Vicki, played by Maureen O’Brien and Steven Taylor, played by Peter Purves do not appear either.

4. The episode came about thanks to holiday leave – Producer Verity Lambert wanted to give the cast regulars an extra week’s holiday between the second and third production blocks, extending their break from five weeks to six, and so it was decided to make the final episode in serial T a one-off story without including any of the regular actors.

5. Both Mission to the Unknown and The Daleks’ Master Plan were the only 1960s Doctor Who stories that were offered for overseas sale but never purchased.

6. An unabridged reading of the book Daleks – Mission to the Unknown by Peter Purves and Jean Marsh, with Dalek voices supplied by Nicholas Briggs, was released by BBC Audiobooks in May 2010. The episode was also reconstructed as an animation in a project led by Ian Levine.