Well, if there’s any good news to be had about a giant, psychic slug attacking you, there is the fact that there is one sure-fire way to stop a slug infestation.
And fortunately, Ma Tyler’s grandson Jack brought it along. As he, his grandma, and Leela are frozen in place by the approaching Fendahleen, the Doctor desperately tries to stop it with Jack’s shotgun. But it’s only loaded with rock salt.
And that works.
Suddenly, this seems easy.
With the humans free, Leela and the Doctor sneak into the room where the former Thea has changed into a Fendahl, infecting the members of the circle and turning them into Fendahleens. If she gets enough of them, she can gain her full power and start the take-over of the world or the universe or England or something. It’s vague. And it’s power can only affect beings who look it in the eye. Do that and you’re done.
Stael realizes this and asks to borrow the shotgun for himself. The Doctor and Leela manage to rescue Colby.
Now comes the way to stop the evil aliens doing evil alien things. That means building some salt bombs with extra rock salt Ma Tyler was carrying. Colby can run the machines that brought the time screen along in the first place, building an explosive charge, and Leela and the Doctor can both collect the skull and lay a ring of salt around the woman formerly known as Thea.
And that works. The priory goes boom. Everyone who isn’t dead, an alien, or a slug monster gets away, and the Doctor and Leela jump in the TARDIS to drop the skull in a supernova.
Well. That was easy. Heck, the Doctor even acknowledges K9 as a “he” and his dog. It’s always nice to have a pet around.
You know, as long as it isn’t a giant slug monster.