The effects for the invisible enemy, or the tear in space, or whatever this is are pretty cool.

The fact that this story takes place in 5000 AD makes me think that this might be the introduction of K9. (at least, I think it was mentioned that he's from the year 5000; I could be mis-remembering)

The premise of this episode seems pretty scary - all it takes is flying through a specific part of space and everyone on your ship becomes possessed by some kind of unknown force.

This is a really effective first episode - Leela's all on her own on an abandoned space station, and she can't trust anyone, even the Doctor.

Tom Baker does a great job at sounding heartbroken at the start of part 2.

K9:SO ANYWAY I STARTED BLASTING

I feel like, between the Doctor and K9, Leela is going to become very frustrated with their inability to explain things in simple terms.

Clones of the Doctor and Leela are going on some sort of fantastic voyage.

The clone Doctor is being a bit too mean to the clone Leela.

I'm curious how they're going to undo K9's possession, considering that he's not human. (for that matter, I just realized that the TARDIS is likely possessed by whatever this is)

I get that they're supposed to be the Doctor's white blood cells, but I find it funny that the actors have to pretend to be attacked by people prodding at them with beach balls.

The premise remains scary, but the true form of the Nucleus looks so dumb that it's harder to take this seriously:

I like how they aren't even being subtle about how the Nucleus costume makes it difficult for the performer to move, as the Nucleus insists on being dragged around everywhere.

And how one of the brainwashed hospital workers reaches out his arm to grab the Nucleus's arm awkwardly, like the actor's thinking "Should... should I do this?"

Given that the Nucleus sounds like it's struggling to breathe, I feel like it's not going to last long at its current size.

Leela's found a way to stop the brainwashed minions of the virus: stab them in the neck!

Surprisingly, the Doctor went along with Leela's idea to blow up the base.

Finished; the last episode felt like a step down from the rest of the serial, but I enjoyed it.