A creature crawls about inside a space vessel. It opens the door to a human, who is in cryogenic suspension. The TARDIS materialises some time afterwards. The Doctor looks at some panels and turns on some energy; he concludes it is some time beyond the 30th Century. Sarah walks through a door, which closes behind her. She finds it hard to get breath and can't open the door. Harry remembers the button he pressed; the door opens, they enter, the door closes and Harry begins to feel the effects of the thin air too. The Doctor sees the controls have been sabotaged - the wiring has been bitten clean through . He fixes the controls, and air supply, in time. The men place Sarah on a couch. They go to fetch a bit of brandy and are attacked by a station defence guard. Sarah dematerialises; she finds herself in a cubicle, which prepares her for cryogenic suspension. The Doctor and Harry manage to turn off the guard and start looking around the station for Sarah, when they realise she has been transported on a local T-Mat system. Harry thinks he saw something crawl into a ventilator; there is a trail of ooze. The station has a data bank consisting of animal and botanic DNA, human thought and several cryogenic rooms with humans. The Doctor marvels at human ingenuity. (This scene is a Who Moment. It captures much of sci-fi thinking on humanity's future of space flight and cryogenics in just a few seconds.) They notice the slime trail again. Harry finds Sarah in one of the cubicles. He looks for a resusitation cylinder; a giant insect lurches towards him when he opens a cupboard.
The insect creature is dead and has been for a while. One of the cubicles starts to operate and the Doctor and Harry meet Vira. She is surprised to see them as they are not among the chosen. Vira revives Sarah and the station leader Noah. Vira explains that Earth has been ravaged by solar flares and that the station was creayed to save humanity. The humans have overslept though because the insect creature sabotaged the controls. A green, maggot creature crawls towards some controls in the solar stack room; this creates a fault which the Doctor rectifies, and which he goes to investigate. There he finds something is in the solar stack. Vira discovers an Ark human, Dune, has vanished. Noah stops the Doctor deactivating the solar stack by shooting him with a stun gun; he then heads for the solar stack, where he is stung by the maggot creature. Sarah and Harry tend to the Doctor; they rush off after Noah; Noah takes them prisoner and back to the cryogenics room. Libri is revived and he starts to shout "get back" because he saw something where Noah is standing. He is starting to act strangely; he says he is Dune and that the revivification must stop. The Doctor deduces that Dune was eaten by the insect creature's larva and that it has assimilated the man's knowledge. Libri follows Noah but is shot dead by the commander, who is beginning to metamorphosise. His left hand is gone and his arm is now a green something!
Noah sends a message to Vira. He tells her that they are in great danger from the Wirrn; she must take charge and return humanity to Earth quickly. Noah tries to fight the alien influence but is losing the battle for his mind; the Wirrn plan to absorb (eat!) all the humans. The Doctor and Vira find Noah, who is rapidly losing his human form. Noah tells the Doctor that the Wirrn is fast approaching its adult phase. More humans, Lycett and Rogin, have been revived. The Doctor wants to find the weak part in the Wirrn's cycle in order to destroy it; they will be crawling all over the station within hours. He takes a retinal membrane from the dead Wirrn to make an image of what it saw; he links up with his brain. (Despite the dangers, he does it to save his favourite species, humans.) The Wirrn larva moves towards them; they shut it out, whilst the screen shows images of the Wirrn queen's last moments. The Doctor wonders why the larvae has begun to attack; he speculates that electricity is deadly - it was the autoguard Harry and he faced earlier. The Doctor uses the T-Mat to move Rogin and Harry to another part of the Ark; there is a power failure and the air supply is cut because the Wirrn don't require air. The Doctor goes to check on the larvae; they have started to pupate. There he sees Noah who completes his transformation into a Wirrn.
Vira and Sarah rescue the Doctor. "Noah" tells Vira to leave the Ark and says how the Wirrn's breeding ground was destroyed by humans. The Wirrn had previously only used non-intelligent herbivores in their cycle; now they will use the humans, assimilate their knowledge and develop an advanced civilisation. The Doctor, on a suggestion of Sarah, plans to use power from a transport ship in order to make a sort of electric fence to protect the humans still in cryogenic suspension. Sarah crawls along a shaft with a cable in order to connect the power from the ship to the cryogenic room; the Wirrn begin hatching and one tries to get to her via a grill. She crawls on, gets stuck but manages to make her way out when the Doctor gives some "encouraging" words; i.e. he irritates her enough so that she wants to give him a verbal. The power is connected and a Wirrn is repelled; it moves off to report to the others. They begin to plan a way to break into the cryogenic room. Noah talks to the Doctor and says they should leave the Ark; the Doctor tries to get him to remember his humanity and lead the swarm away, leaving the humans. The Wirrn begin to space walk towards the cargo hold and spaceship. Rogin sacrifices himself to send the Wirrn into space in the ship. Noah radios a goodbye to Vira; he offers himself to kill the Wirrn. The Doctor, Sarah and Harry go to Earth to repair the Ark's T-Mat reception.
One of the problems with the story is its implicit eugenics. The Doctor, Sarah and Harry are viewed as dangers to the Ark's genetic pool. (The humans aboard have been chosen for their superior DNA. ) This aspect of the story seems rarely touched upon, judging by reviews I have read.
The Wirrn, both larval and adult, are not well realised. However, if one is prepared to let that one pass, the idea of basing an alien creature on the macabre life cycle of some wasps, with us as the food and host, is good. Furthermore, the action takes place on a space station which allows claustrophobic elements to be brought into the script. This story was broadcast in January 1975 - some four years before the classic "Alien" film by Shusett and O'Bannon was at the cinema.
The Ark is a lovely set, with its white interior and bright lighting. The music by this time had long since moved on from the electronica of Pertwee and had developed a different quality, which I call (surprise!) the early T. Baker sound. That music certainly gives more atmosphere to stories.
Harry is a right old chavaunist towards Sarah. In a way, that allows Sarah to's feminist side to show itself, although Liz Sladen actually has played down the Womens' Lib aspect of her character; she preferred to think Sarah was somply a strong individual.
An enjoyable story.
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