After their skirmish with the Daleks, the TARDIS crew hurriedly leave the planet Skaro in an attempt to return to contemporary England, home to schoolteachers Ian and Barbara. However, an explosion renders both of them, plus the mysterious doctor and his granddaughter Susan, unconscious - and when they awake, all four sense that something is very wrong indeed. Aggression, paranoia and a feeling of despair have gripped them all - no one seems to trust the other and, bit-by-bit, the Doctor's trusted ship seems to be breaking down around them. But is it? Has the TARDIS finally tired of its inhabitants, or is there a more logical, more rational explanation for everyone's behaviour and the ship's bizarre course of action?