The Edge of Destruction

As they slowly recover from the shock of being thrown to the TARDIS floor, the Doctor, Susan, Ian and Barbara all seem to be acting strangely. It gradually dawns on the travellers that what they have been experiencing is an attempt by the TARDIS itself to warn them of something. The Doctor finally realises the fast return switch he used when leaving Skaro has stuck, and the ship has been plunging back to the beginning of time and its own destruction.