Another adventure for everyone's favourite Time Lord. The Doctor (Tom Baker), Sarah and Harry land on space station Nerva, where the chosen few of the human race are in suspended animation, awaiting the call to revive. However, an insectoid alien life form known as the Wirrn has infiltrated the Ark and cut through the humans' alarm clock. The Wirrn now intend to use the sleepers as the hosts for their hatchings and claim the Earth as their own.