The Doctor and Peri land in desolate ruins on Earth, tens of thousands of years in Peri's future. The Earth had been attacked by the Pararachnids, huge space-borne swarming aliens who left the planet in ruins and incidentally left several colonies of Pararachnids behind. The humans that survived the attack built tunnels underground and set up the Line, a place where they were put in a kind of suspended animation and were fed and exercised by robots called Monitors. The Doctor came to this time to find an item he needs to recalibrate the TARDIS.