The harsh British winter of 1963 brings a big freeze that extends into April with no sign of letting up. And with it comes a new, far greater menace: terrifying icy creatures are stalking the streets, bringing death and destruction. The First Doctor and Susan, trapped on Earth until the faulty TARDIS can be repaired, are caught up in the crisis. The Doctor seems to know what is going on, but is uncharacteristically detached and furtive, almost as if he is losing his memory... Susan, isolated from her grandfather and finding it hard to fit in with the human teenagers at Coal Hill School, tries to cope by recording her thoughts in a diary. But she too feels her memory slipping away and her past unravelling. Is she even sure who she is any more...?