Hey! Wait a minute! I can see that enemy! He isn’t invisible!
OK, so, the Nucleus grew to man-sized, looking like a giant peeled shrimp. The Doctor is also somehow cured, so he can exchange some banter, but the Nucleus still wants the Doctor in the collective and has him wheeled off.
Leela, with a painted face and a borrowed uniform, easily rescues him. These infected people are dumb.
But Lowe, the Nucleus, and all of the remaining infected in the hospital leave, having Marius stay behind to find the Doctor. They’re flying off to Titan to finish the spawning process.
Now, the Doctor doesn’t necessary disagree with the Swarm’s existence. He just believes in balance. As such, he wants an intelligent way to fix everything, maybe something better than Leela’s constant suggestion to just blow the whole fueling station up. She’s a direct one. Also, her clone left the Doctor with his immunity, so he can cure Marius and make some antibodies to stop the Swarm. That seems like a good plan. The Doctor, Leela, and a borrowed K9 take the TARDIS back to Titan.
Things are bad there. The infected are now mostly blaster-proof, the Nucleus has bred a bazillion babies, and Leela’s back-up plan of “stick a knife in a bad guy’s neck” is only effective if she can repeat it thousands upon thousands of times.
As for the antibodies, the Doctor lost them when he and Lowe wrestled a bit and Lowe fell into the Swarm pit to be presumably eaten. Why not cure these guys too, Doctor?
But the Doctor does have one last plan, involving a blaster, a bit of string, and turning up the oxygen content of the station. After setting that up, he just taunts the Nucleus enough to smash its way out of a metal booth. Fortunately, that takes awhile, and the Doctor rushes out to the TARDIS so fast he initially forgot Leela and a depowered K9 were stuck outside of it.
Don’t worry. He picked them up before the Nucleus smashed its way out, set off the blaster, ignited the oxygen, and blew up the base with Titan’s methane atmosphere.
That was Leela’s idea after all.
In the end, the Doctor goes to return K9, but Marius is going back to Earth soon and offers to let the Doctor keep him as another companion.
Considering how often K9 lost power and couldn’t do much, we’ll see how that works out.
Then again, K9 was apparently an expensive prop that may or may not have broken down a lot, so the BBC might have wanted to get their money’s worth on that thing. Plus, I like robots, I like dogs, and I kinda remember K9 from when I was a kid and my dad watched this show.
Let him stick around for a while.