Leaving Earth and UNIT behind, the Doctor, Sarah and Harry depart in the Tardis and wind up thousands of years in the future, on a space station filled with the last of humanity, cryogenically frozen, orbiting the Earth, awaiting their wake-up call.
Obviously, things go wrong: the humans have overslept and an alien lifeform is oozing itself around the Ark, infecting humans and turning them into hosts for its offspring, resulting in some rather excellent and scary make-up effects, and some rather rubbish looking insect costumes.
This is a strong serial on its own terms. The close-quarters environment and small roster of characters in a perilous situation makes for compelling viewing, and pleasingly wraps itself up in just four parts. However, the trouble with having seen every episode is that some of the concepts start getting recycled. Aliens infecting humans in a spaceship has already been done countless times, and the whole ‘ark full of the last humans’ idea was already done rather brilliantly in the Hartnell four-parter The Ark, which had the added bonus of a time-travel-induced twist.
So, it’s the execution that matters, and this was a more standard monster story, with some interesting ideas, like the monsters (the Wirrn) being able to accumulate knowledge from their hosts and pass it to their offspring. The Ark sets were also pretty good with a sterile futuristic aesthetic, which is matched by the cold and unfeeling personalities of the chosen humans.
The Doctor is, again, a pleasure to watch. He’s never patronising, he’s quite blunt with people about their prospects for survival but he does it with a cheerful smile. He also has some good banter with Harry; the part where they’re trapped behind the table with the sentry gun is good fun. The cast of companions is more rounded now, but this has put Sarah Jane in the position of the “scared girl” who nobody listens to and gets captured (aside from the end where she helps run a cable through a tight conduit). I hope this doesn’t continue. Why should Sarah be panicking while Harry is calm and cheerful about the situation? It should be more of a shock to him.
The story ends with the Ark saved and the aliens ejected in a shuttle, but the Doctor’s work is never done, as he ‘beams’ down to planet Earth with Sarah and Harry to check out its viability for life, leaving the Tardis in orbit.