They materialize inside of a space ship after Harry messes with the controls, thus the Doctor doesn't know exactly where or when they are. Turns out they're on Space Station Nerva an Earth satellite constructed in the 30th Century. They discover a number of people in suspended animation and the Doctor susses out that they are the remnants of the Human Race, as the Earth had become uninhabitable due to solar flares some thousands of years previously. They were scheduled to be awoken when the Earth was inhabitable again and would re-populate the planet by flying their space ship the Ark, down to Earth. But something tampered with their "alarm clock" and they failed to wake up.
As our travellers investigate they sleepers start to wake, first is Vira a medical-technician who assists the others. Her commander is "Noah" who explains that the sleepers were the "best' of the human race and that there must be no dilution of the species, so the Doctor and his companions must die. But before he can dispatch our heroes the Doctor discovers that they are not alone on the station. Helps that they had found a six foot tall space fly earlier.
The space flies, dubbed the Wirrn, want to use the space station and the Ark (two separate entities by the way) as a hosting and birthing chamber for their young. Of course it's not quite that easy, the Wirrn's larval form actually takes over a human host and Noah is the first to be subsumed. The Doctor outwits his enemies, saves most of the Ark inhabitants, and the sleepers are free to return to Earth via a Trans-Mat beam, except that the beam isn't working correctly, so the Doctor offers to beam down to Earth to fix the receptors, and the story ends on a cliff-hanger.
Although story wise this serial is not at all very original nor all that interesting, there are several components which must be addressed. On the positive side, Space Station Nerva and the Ark look pretty darn good on the inside, and the models used for the exterior shots aren't badly made. The design of the sleeper chambers is especially pleasing, as it's obviously done on a budge (the 'cut outs' for the sleepers are clearly styrofoam, but the canopies that cover them are really cool looking vacu-formed plastic). Unfortunately space vessels do look a bit dated, and in fact have been updated in the early 2000's DVD release with CGI exteriors. The costumes that the sleepers and later Sarah Jane wear are also quite nicely designed and look especially good on the actors who are, after all, playing the best that humanity has to offer.
As with most Doctor Who serials there is a bit of social commentary on the negative aspects of social eugenics, and also a bit on the rights of all beings to survive. Unfortunately there is also a couple of six foot tall Space Flies which are obviously men in badly designed fly suits.
Both Harry and the Doctor treat Sarah like a foolish school girl yet again, and don't even seem to be concerned all that much when she goes missing in an early sequence. The Wirrn, while not necessarily 'villains' in the strictest sense of the word (they're only trying to kill the humans because humans had killed off most of their race) aren't really all that dangerous, except at the end when they 'swarm.' I also got the strange feeling that Ridley Scott had seen this episode while he was working on Alien because several of the sequences and plot devices seemed awfully familiar, especially the sequence where Sarah Jane crawls through a maintenance duct.
On the whole, not a bad episode, and it generally improves on the previous story, but not by a whole lot. Seems to me like the Fourth Doctor stories were a bit weak in the beginning and would take a few go-rounds before they would gel into classics.
Next Time: The Attack of Mister Potato Head