The pacing of the four episodes is perfect, there are a lot of ideas being played out here.

The Monoids also make their mark as the second ever genuinely interesting looking alien race in the show, it took over 100 episodes - but they did it!! I'm surprised these guys never came back in a serious way as their design is magnificent.

The eco-system on a spaceship its refreshingly sci-fi after the previous story. The Dystopian future in which the Earth is about to be burned up, and on top of this an 'ancient' illness will kill the remaining humans is such a daring and forward thinking concept, bringing to mind white plagues brought to indeginous populations. It is up to The Doctor to sort out a mess that he has brought to this culture.

The resolution to the cliffhanger in part 3 is a revelation. Everything seems to rap itself up, they resolve the situation then find themselves 700 years in the future in the same place. There is a giant statue of a Monoid instead of what should have been a human. Brilliant! The Monoids are scarier and more malevolent 700 years in the future and they have taken power.

After Steven's moral outrage at the end of the last story it would have been positive if they'd returned to that issue, as he has gone from "storming out forever" to never mentioning it again.

The alien force end up being their own undoing, turning on each other. Thankfully there is a moral cautionary ending, where the humans are made to understand that they too enslaved the alien race, giving motivation to the monoids to act in the way that they did.

It ends with another great cliffhanger with The Doctor suddenly fading - what's going on - who is attacking them?