Doctor Who: The Edge of Destruction (1964)
Wedged between the epic story of The Daleks and the upcoming epic of Marco Polo (for which, sadly, no episodes survive), The Edge of Destruction is a cheap little time-filler in which nobody leaves the ship, and no additional characters beyond the regular crew are involved. With the stories either side of it going over budget, and with a delay in the production of Marco Polo, there was the need for exactly this sort of story, and it was written at short notice.
That said, however, it is actually quite an important tale in the series, and it introduces a number of concepts which helped to expand the mythology of the show. Quite simply, as they are leaving Skaro after their encounter with the Daleks, the TARDIS shudders to a grinding halt, throwing everybody aboard violently off their feet, and delivering them to the Edge of Destruction.
Barbara is the first to recover, and she stumbles around the interior of the control room in a daze. She spares barely a glance for the unconscious figure of the Doctor on the floor, bleeding from a head wound; she seems slightly more concerned for the welfare of Ian, but doesn't quite seem to recognise him. He is slumped in a chair, also unconscious.
Susan is the next to recover; she is more concerned about the state of her grandfather, but barely seems to recognise Barbara, and apparently doesn't know Ian at all. Ian's recovery is portrayed quite startlingly; he goes from slumped in the chair to standing erect, seemingly in the blink of an eye, off-camera. His behaviour can only be described as odd.
Finally the Doctor comes to. His wound is bandaged (with a pre-medicated bandage; the coloured stripes of medication will fade as it is absorbed, and by the time the bandage is white, the wound will be healed.) As she returns with the bandage, however, Susan screams: impossibly, the TARDIS doors are open.
With everybody behaving strangely, with gaps in their memories, the question is obvious: has something come aboard with them? Is something perhaps hiding inside one of them? The Doctor and Susan are both suffering from painful headaches; have they been attacked by Ian and Barbara? Suspicions fly, paranoia grows, ugly accusations and threats are made. Susan waves around a wicked-looking pair of scissors in self-defence -- but in a fit of madness (with a clearly Psycho-inspired music riff) she stabs them into the couch. Stabbity-stabbity-stab!
Then the fault-detector panel sounds an alarm that is very much a precursor of cloister bells to come -- it seems that every system in the ship is failing. They are out of time. Furthermore, it seems that time itself has been taken away from them. They are on the edge of destruction, the brink of disaster, and the Doctor seems helpless to prevent certain catastrophe. He doesn't have the first clue about where to begin.
Or does he?
This episode gives us a barest sliver of a glimpse of the life of the Doctor and Susan before they came to that Totter's Lane junkyard in London; we see an image on the monitor that the two recognise as the planet Quinnis -- "where we nearly lost the TARDIS, four or five adventures back." (That adventure is explored in greater detail in the audio adventure of the same name -- which I'll get to in the fullness of time!)
It also shows us a little more of the ship beyond the control room, and answers the rather mundane question of where our travellers sleep (in fold-down beds with zero privacy!) Left unanswered is the slightly more important question of where they go to the toilet? We see a little more of the food-dispensing machine, which can produce both food and drink -- although as this story unfolds it seems to be malfunctioning in odd ways. (Is this a clue?)
More importantly, it introduces the idea that the heart of the TARDIS lies somewhere beneath the central column -- and suggests, indeed, that the rising and falling of the column during flight is very much the heartbeat of the ship. (The heart of the TARDIS will have an important role 45 years later, as the Ninth Doctor's companion, Rose, will discover...) Furthermore, it gives us a look at some of the gaps in the Doctor's knowledge; he clearly does not understand some of the abilities of the TARDIS.
This story plays an important role in bringing together the two separate factions aboard the ship. After being kidnapped, and then dragged into mortal danger by the Doctor's capricious behaviour on Skaro, the two humans are understandably mistrustful of the Doctor; in return, the Doctor is suspicious of their motives (and accuses them of sabotaging his ship -- a fine thing to say after his own games with the Fluid Link in their previous adventure -- to get home. Somehow...) By its end, they seem to have mostly reconciled, the Doctor even going so far as to, well, perhaps "apologise" would be going a little far, but in his own way he manages to express regret for some of the things he might have said... Bridges are mended, walls are torn down, etc...
And how do they escape their predicament? What has brought them to the edge of destruction? Well, that would be telling -- but let's just say it turns out to be something delightfully prosaic.