Separated from his companions, the Doctor attempts to find solace in the history of his favourite planet – Earth – but instead discovers new threats lying in wait. Travelling from twentieth-century East Berlin to sixteenth-century Strasbourg, the Doctor encounters creatures from other realities: monsters beneath the waves, and human beings determined to exploit their fellow man. But how long can he survive without a friend? Deep beneath the streets of East Berlin, Peter Meier patrols the border in an old underground station. But when the TARDIS materialises nearby, Peter realises he is far from alone.