Huh. I think I know why people hate Adric so much.
Adric spends a lot of time in this episode saying being a vampire might be rather cool, and since he’s due to be changed over while Romana is there to be a sacrifice to the Great One, the giant original vampire that fought the Time Lords and was the sole survivor of his side, the thing that made the other three into human-sized vampires, and who probably needs more than a handheld stake to take down.
Of course, it comes out later that Adric, for all the trash he talks about the Doctor, is actually just playing along until there’s a good time to escape. But still, he does seem like something of a sniveling snot of a kid.
But the Doctor needs to save the day first. For that, he’s gonna need help.
First, there’s the rebels. They think the Doctor ditched them. As if! He zaps the TARDIS right into their meeting room and takes over. He needs numbers to distract the guards, rewire one of the towers to take off, and then use that to take out the Great One. Because the castle was once a spaceship, and he thinks it can fly under the right rewiring. Of course, the Doctor himself will be too busy to lead the rebels. He’ll leave that to the one fellow he knows who is immune to mind control and has a nose laser.
Yeah, K9 will actually be useful in this serial’s finale.
So, everything more or less goes according to plan. The Doctor successfully activates a tower, Adric and Romana slip away, and just as the Great One bursts out of the ground, the pointy-shaped tower comes back to the ground and apparently stakes the creature through the chest. As thus ends the Time Lords’ ancient enemies because killing the head vampire somehow also kills the other three as they rapidly grow old and die before they have their revenge on the Doctor.
So, all that being done, the Doctor promises to take Adric home in the TARDIS before too long, but we all know by now that’s a lie. Especially since the next serial is the last one for now for both K9 and Romana.