The one where Helen A's empire comes tumbling down...
"There are no other colours without the blues."
"There are no other colours without the blues."
The Doctor, rather than try to rescue Ace from the miracle survival act at the Forum, forgets the fact his best friend is in danger and decides instead to hang out at Forum Square while he waits for Earl. He passes the time by chatting to Trevor Sigma, who shows him just how many routine disappearances there have been on Terra Alpha in the last six months - how can there be anyone left?! He also ends up crooning jazz standard As Time Goes By into a microphone.
As much as I love this story, it's messily constructed, as demonstrated here when the Doctor's movements do not follow on logically. After expressing concern for Ace's welfare at the Forum, he then drops that concern altogether to move to a different strand of the story. This serial could have done with four episodes and a bit of a rejig.