Theory:Doctor Who television discontinuity and plot holes/The Edge of Destruction


 * The Doctor tells Ian that they only have 5 minutes to survive in episode 2 - yet it takes over 5 minutes before they release the fast return switch. The Doctor even takes time to deliver a long and rather pointless speech when he knows that their end is only a few seconds away.
 * Time has since been shown to pass differently inside the TARDIS. As for the Doctor becoming extremely excited about a scientific phenomenon, that is completely consistent with his character, as we have since seen.
 * The five-minute (or worse) estimation of their survival seems to be a conclusion the Doctor has reached from the system-wide failure on the fault locator (implying that the whole ship is on the verge of disintegration). It later turns out that this fault signal was merely one of the TARDIS's increasingly desperate attempts to communicate with the travelers (therefore not actually indicating the ship was already falling to bits around them, but hinting that it could very well do so if they kept on ignoring a certain jammed switch ...).
 * Furthermore, once he and the rest of the crew pin the fault down to the Fast Return Switch, they move to fix it as fast as they can. Before this, though, he and Ian both believe the TARDIS to be doomed. The Doctor fairly explicitly indicates to Ian that they're all going to die, and that the two of them should simply try to appear busy, making progress, talking- giving Susan and Barbara false hope so that they won't despair, and will receive an instantaneous and unknowing merciful death, rather than cowering waiting for the end.  During the long, rambling speech, the Doctor is probably still expecting to be annihilated at any second- but, how would he prefer Susan to die? Watching her grandfather excitedly revelling in a scientific phenomenon and with hope and confidence that any moment he'll find a way to use it to save them, or in terror and despair, waiting hopelessly for an inevitable end?


 * William Hartnell, completely throwing the other actors during one scene by saying the same line ("It's not very likely") twice, and fumbling "You knocked both Susan and I unconscious". He also omits the scripted explanation for the melted clocks.
 * The Doctor, like many people, sometimes stumbles over his words.
 * The line he repeats is actually "It's not very logical" and Jacqueline Hill gives the scripted response to it the second time around - the Doctor may just be provoking Barbara with the word "logical" until she calls him on it.


 * The fast return switch should have sent them back to 100,000 BC as that was the last place it visited prior to Skaro and not 1963 London.
 * The Doctor was shown to have frequent lapses in memory at this point in his life.


 * Although various later (non-television) stories have claimed the fast return switch returns the TARDIS to its previous location, this is pure fan lore. The Doctor's explanation of the switch here is simply that it sends the TARDIS back in time.
 * How do we know that he hasn't pressed the switch twice, as in later stories such as in Seasons of Fear, to return back to a previous destination he tells Charlie to press the fast return switch three times, which takes the TARDIS back from the Time Vortex to Roman Britain.


 * Since the spring was broken, the TARDIS could have been continuously going back in its history. Much like repeatedly hitting the back button on the browser.  If the TARDIS history is pre-initialized with '0' for the time before the first location, then the TARDIS would head towards the Beginning of Time.  Fixing the problem before reaching that would just land the TARDIS at some location the TARDIS had been to before.  In this case it just happened to end up at one of the times the TARDIS landed in 1963 London before Skaro.


 * If the Fast Return Switch can take them back to previous destinations, why don't they try to use it again? Even if they were worried it might get stuck again they could just check it.
 * Perhaps they did, and found it in the same wretched condition as the rest of the 1960s-era TARDIS control system. In retrospect, it seems that the Third Doctor's enforced exile was the best thing that could have happened to the TARDIS, as it resulted in the extensive dry-dock overhaul it was sorely in need of.


 * Alternatively of course, the TARDIS is alive, and the Doctor's bungling breaking the switch nearly killed her as well. Perhaps he did try to use the Fast Return Switch again after the four of them got back aboard at the end of Marco Polo (TV story), only for the Ship to spell out "Not Bloody Likely" in melted clock faces all round the console room, and to use her Architectural Configuration systems to hide the Fast Return switch somewhere the Doctor wouldn't find it again for a very long time.


 * Considering how on this occasion the Fast Return Switch almost drove everyone on the ship made before hurling them into the beginning of time and killing them all, they're probably a little wary of trying it again. Quite understandable too; it broke once, there's no guarantee it won't break again.


 * At one point, Ian checks the Doctor's heart, and claims it to be alright. The Doctor doesn't act like one of his hearts has stopped, so it being alright should've been a double heartbeat that would've been strange to Ian.
 * The first Doctor only had one heart


 * Why does Susan go berserk and start threatening Barbara and Ian with scissors, and generally alternate between a cold, sardonically alien personality who doesn't trust them and sides with the Doctor, and a scared, frightened girl who sides with Ian and Barbara?


 * Because the former personality isn't Susan. This story is written by David Whitaker, the same story editor who commissioned, and presumably edited, The Sensorites (TV story), in which Susan's telepathy is a significant plot point. The 'evil' Susan in these two episodes is the TARDIS herself, finding herself hurt, not knowing why, finding herself being pushed on a course to self-destruction and not knowing why, unable to communicate adequately with her crew to ask, and, like the Doctor, instinctively inclined to blame the alien 'strays' that he and Susan have brought home, or at the very least to insist on an account of herself. When Susan hysterically attacks and stabs her bed, this isn't just a breakdown- that bed is part of the TARDIS. She's trying to physically externalise the internal effort at fighting off the being which has been using her to communicate.


 * Also; the whole point of the story is that what's happening is having a severe negative psychological effect on the TARDIS crew. Susan's breakdown and bipolar personality shifts are symptoms of this effect. If the characters weren't acting unusually or irrationally, there'd be no story. It's not a discontinuity, it's part of the plot.