This is a really cool Doctor Who episode, one of the first with the inimitable Tom Baker (who also happens to have some of the greatest reaction shots I've ever seen, movie or TV or otherwise), as it deals with a swarm of larva-hatching-in-bodies insects on an "ark" out in space. The ark, as we learn, was created for the purposes of making sure there were some members of the human race who survived following a terrible event on Earth some thousands of years ago (the Doctor isn't quite sure what year it is when he arrives with Sara and his other friend, but it's certainly many thousands of years, "past 30th century"). While one might expect there to be more women on board to help the future of the human race procreate, or that there would be more multi-ethnicity, it's still a fantastic premise that surmises what's good in human beings to persevere... but what can also be unexpected to corrupt it.
The bugs on display here, as well as the slimy larva counterparts, are of course cheesy and silly and all that and a bag of Doctor chips, but I somehow didn't mind them after a short while. In fact, they're probably more convincing than if they were done today with chintzy Sci-fi channel type CGI, as we can at least see these creatures for all their ten cents are worth. And the essential conflict of the piece, from a script by Robert Holmes, is very well crafted and giving Baker plenty to do (within his range as the Doctor) in a short stretch of time - including strapping a device to his head to read the mind of one of the greenish slug things, at his own cost! I also loved the whole idea that Noah himself, on this 'ark', would suddenly turn into the chief villain and possible annihilator of the (apparently not quite) last hopes for humanity. There's even an awesome bit with Sara climbing through a vent, with the Doctor supporting (or rather helpfully berating) her through to the other side.
Like the best of Doctor Who, it's witty and clever without going too high-falutent, and the special effects and creatures are wonderfully dated.