The Doctor, while attempting to correct the TARDIS's faulty navigation circuits, causes a small explosion. The Doctor, Barbara, Ian and Susan are all temporarily rendered unconscious. After they awake, Ian and Susan appear to have slight cases of amnesia and everyone begins to act strangely. Unexpected events are happening on the TARDIS, the travellers are becoming suspicious of each other's motives, and the Doctor even accuses Ian and Barbara of sabotage. Fearing that they have been taken over by some alien force — or that they have intentionally sabotaged the TARDIS in order to force the Doctor to return them to 1963 — he drugs Barbara and Ian unknowing that Ian is also suspicious and has not taken the drink given to him. The Doctor attempts to explore the problem without interference.
Gradually it becomes clear that the strange events are an attempt by the TARDIS itself to warn the crew 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 malfunction is causing the TARDIS to head back to the beginning of time; the strange events were just the TARDIS's attempts to warn its passengers before the ship is destroyed. Fixing the switch brings all back to normal. Although the day is saved, Barbara is still affected by the Doctor's harsh words earlier. The Doctor is forced to do what he least enjoys — apologise, and admit that he was wrong about Barbara and Ian.
The story closes with the TARDIS materialising on a snowy landscape, where Susan spots a giant footprint in the snow.