The TARDIS arrives in the far future on the Ark space station where mankind's survivors are in hibernation. Sarah is accidentally placed in suspended animation.
When the Doctor restores the power to the Ark, the humans begin to revive. But the station has been invaded by the giant insectoid Wirrn.
The Doctor subjects himself to a dangerous experiment so that he can access the memories of the dead Wirrn Queen. Elsewhere, the larvae Wirrn are pupating into their adult form.
The mutated Noah has now fully transformed into a Wirrn adult and intends to allow the Wirrn to devour the sleeping humans, absorbing their memories and knowledge.