Sylvester McCoy (The Doctor), Bonnie Langford (Mel), Richard Gibson (Mitchell), Pamela Miles (Bailey), Francis Magee (Stewart), Audrey Schoellhammer (Reed), Trevor Littledale (Potter), Trevor Martin (Professor Capra), Daniel Hogarth (Slithergees).

• On the White disc, while explaining the hazards of time travel to Mel, the Doctor compares encountering effect before cause to tripping over one’s own footprints. At the equivalent point on the Black disc, while explaining the hazards of time travel to Mel, the Doctor trips over (ahem) something which is never explicity identified.

• If the Quarks menacing the space yacht Pinto are the same as those seen in The Dominators, then the natural conclusion would be the yacht is under attack by the Dominators themselves, since the Quarks were no more than their robot servants. However, this is more likely to be a reference to the comic strips of the late ’60s, in which the Quarks themselves were touted as a race of evil conquerors. One can always conclude that the Quarks developed self-awareness, overthrew their masters and struck out on their own, but frankly that sort of takes the fun out of it.