Aboard a space station orbiting Earth, some alien creature approaches a human-shaped compartment and opens it. Some time later, the TARDIS materialises in a dark room aboard the same station. Harry accidentally touched the controls and is apologising profusely – he's bumped something and randomized the TARDIS's flight plan. They're in a sealed compartment and running low on air. Harry opens an adjacent chamber and Sarah wanders in as the door closes behind her, sealing her in.
The Doctor concludes the station is thousands of years old, beyond the date of its creation in the 30th century. Harry and the Doctor finally notice Sarah is gone and gets Harry to recreate the sequence that opened the door. Sarah is unconscious and when they go in after her the door closes again, trapping them in. The Doctor finds the oxygen supply but it's non-functional along with many of the other circuits – something has bitten through the wires. Harry and Sarah pass out and even the Doctor feels the strain, but he manages to get the power restored and the oxygen flowing. They put an unconscious Sarah on a couch and get the door open, then go to get some brandy for Sarah. An automatic self-defence system activates and opens fire on them, while Sarah is teleported off the couch.
The Doctor concludes the system was deliberately cut off by whatever chewed through the wires, and his repairs have re-enabled it. Sarah is in a biocyronic preservation system and an automated voice tells her to stay still while it prepares her for processing. The Doctor loosens the screws on the table they've hidden behind and the two of them use it as an inorganic shield so they can get to the cutout switch up on the wall. The Doctor tries to loop it with his scarf without luck, and then Harry tries a cricket ball with a similar lack of success. Harry throws his shoe as a distraction giving the Doctor enough time to throw the switch. Elsewhere Sarah's chamber is sealed and cryonic gas is pumped in.
The Doctor realises Sarah has been transported away and concludes she's only a short distance away. They go into the station proper and find a sterile area that opens for them. Harry spots something ahead of them and they investigate to find it's a slime trial, left by some kind of mollusc. They go through a decontamination chamber and then a room containing cryogenically preserved animal and plant specimens and a microfilm library with all human knowledge. The Doctor concludes they are in some kind of "lifeboat" and wonders where the missing element is – man itself. Another door opens revealing a chamber filled with frozen humans waiting to be awakened.
Harry is skeptical but the Doctor points out that the humans are perfectly preserved and says they must find Sarah and leave the frozen people to waken up. Harry spots another slime trail that leads to one of the storage compartments. They examine the cells and find Sarah frozen in a chamber. Harry goes to find a revival unit and opens a closet… where a giant insect falls on him.