The TARDIS materialises on board a space station. Everything is fully automated, including defence systems, and the station seems to be deserted, until the Doctor and Harry find a room full of humans held in suspended animation - and also the body of a dead Wirrrn.
One of the bodies is missing, and when of the people are revived, the Doctor and Harry are accused. Meanwhile, Sarah laid down on a bench, and was teleported to the cryogenic facility.
Investigations by the Doctor find that the station has been invaded by the Wirrrn. The Wirrrn had damaged the machinery that controlled the de-freeze process, hence the people failed to awaken at the right time.
The Wirrrn had been using the people as food. The humans' leader, Noah, was assimilated by the Wirrrn, giving them access to all his memories. The Doctor used this to convince Noah, who was now a Wirrrn himself, to take the Wirrrn outside the space station. When the shuttlecraft's engines were fired the Wirrrn were destroyed.
The original idea was for the people on board the space station to return to Earth when the solar flares had calmed down. The transporter was not functioning properly, and the Doctor offered to go down to Earth and fix the problem.
