The one where Jamie and Zoe become fictional characters...
POP! That's the sound of my little brain trying to cope with the mind-twistingly meta idea that Jamie and Zoe, two fictional characters from the BBC Television series Doctor Who, have just been turned into fictional characters within the fictional universe of the programme. They're doubly fictional - fictional for BBC viewers, and fictional in-universe too! I love it; I love the devil-may-care courage this script has and the bracing cavalcade of ideas that spill out of it. Peter Ling really should have been employed again by Doctor Who; it's s shame he wasn't.
There may be a good reason for why he wasn't though - the production office was in turmoil at the time, as commissioned scripts were falling through hand over fist. This explains why The Dominators was reduced from six to five episodes (gawd, imagine another episode of The Dominators!), why The Mind Robber had a fifth one added (written uncredited by script editor Derrick Sherwin), why you suddenly had longer serials (an eight-parter and a ten-parter), and why newbie Robert Holmes was commissioned twice in five months!