OK, so, Marshmen everywhere inside the Starliner, a spaceship that has been under repair for at least 40 generations (and probably much longer), Romana is now a mute helping the Marshmen, and even if the ship were working, it lacks a competent pilot.
The Doctor is on the case, so it’ll be a piece of cake.
It helps that the Marshmen seem more inclined to just knock random stuff over rather than hurt anybody. Sure, they do attack a few people, but for the most part, they don’t seem to difficult to evade. Heck, one corners the Doctor at one point, but the Doctor’s spots K9’s severed head on the end of the Marshman’s club. Using that as a mask of sorts, he manages to sneak out past all the Marshmen plus Romana.
And then he makes the most unbelievable discovery while locked away in a lab with Adric and his remaining teenage rebel friends: the Marshmen, the people of the Starliner, and the Marshspiders all have the same DNA. He even has a formula mixed up to cure Romana while he’s at it.
Yes, it seems the Starliner has been there a lot longer than anyone thought. Only some of the Deciders knew the truth: the Starliner’s inhabitants can’t go back to the planet the Starliner came from because they’ve never actually been there. Sure, they can still go. They just can’t return.
I love semantics sometimes.
It seems after the initial Starliner crash, some Marshmen evolved to look and think just like the Starliner’s original passengers and have been living there ever since, most of them having long forgotten their actual origins.
That’s actually rather clever sci-fi right there.
And then, by sheer accident, the Doctor learns pure oxygen puts the Marshmen to sleep, allowing him to knock out Romana and drive the Marshmen back into the Marsh. He gives Romana the antidote, gives the remaining Deciders a rough lesson on how to fly the Starliner (it works), and then he and his companions are off in the TARDIS for whatever comes next. K9 seems to be repaired again, and then he and Romana find a piece of tech that Adric had on him, so that kid is probably hanging around somewhere onboard. Since his brother was one of the few known casualties of the Marshman invasion, I guess they figured he had nowhere else to go.
Let’s see how soon I get sick of him then.