An Unconscious Doctor

The Doctor, while attempting to correct the TARDIS's faulty navigation circuits, causes a small explosion. He and his companions are all thrown to the floor as the control room is plunged into darkness.

The four time travellers slowly, one-by-one, regain consciousness to find the Console Room in semi-darkness. As they start to recover from the shock of being thrown to the TARDIS floor, The Doctor, Susan, Barbara Wright and Ian Chesterton all seem to be acting strangely. The Doctor has sustained a nasty gash to his forehead and the others are suffering memory losses and pains in the head and neck.

Tensions start to mount as a number of unexpected and unexplained events start happening to the TARDIS. The main doors open and close on their own accord and a confusing sequence of images appears on the scanner screen. Even Susan acts strangely when she tries to stab Ian with a pair of scissors.

Barbara and Ian Tend to The Doctor

As these strange events increase the travellers become more suspicious of each other's motives. While Barbara and Ian contemplate the possibility that an intruder may have got on board, The Doctor is convinced that they have sabotaged the instruments with the intention of blackmailing him into returning them home. Even Susan comes to mistrust the two teachers until eventually she realises that they could not possibly have been responsible for everything which has happened.

Barbara though angrily refutes The Doctor's suspicions by reminding them all that, during the recent events on Skaro, she and Ian risked their lives to save The Doctor and Susan from the Daleks. Barbara then notices that the clock The Doctor keeps in the Console Room has melted, a sight which horrifies her.

An Unwell Susan

As tensions increase even further and The Doctor tries, without success, to locate the problem they start to fear that some alien force may have entered the TARDIS. As the danger intensifies the TARDIS then experiences a total systems failure. The Doctor realises that the TARDIS's power source, located beneath the console, is trying to force its way out and that they are only minutes away from destruction.

Faced with a common peril, the travellers forget their differences and begin to work together. It then gradually dawns on Barbara that what they have been experiencing is an attempt by the TARDIS itself to warn them that something is wrong.

Thanks to Barbara's clue-gathering, The Doctor traces the problem to a broken spring in the Fast Return Switch. The Doctor discovers that it has become jammed and that the TARDIS has been rushing backwards through time towards the very beginning of the Universe. The strange events were just the TARDIS's attempts to warn its passengers before the TARDIS was destroyed.

Susan by the Food Machine

With the fault repaired, and the ship returned to normal, The Doctor is forced to admit that Barbara's intuition has saved the day and that he has to make some apologies. Although the day is saved, Barbara is still affected by The Doctor's harsh words and accusations that he made earlier. The Doctor though has realised that he was wrong about Barbara and Ian and that they can be trusted.

As the TARDIS lands once more Susan and Ian exit the TARDIS and find that they have arrived on a snowy landscape. Susan then spots a giant footprint in the snow.