Doctor Who: The Edge of Destruction by Nigel Robinson

clue...’
    ‘Perhaps we’ve been given nothing else but clues...’
    Everyone turned to look at Barbara.
    ‘What do you mean?’ asked Ian. ‘Like the food machine registering empty when it wasn’t?’
    ‘Yes,’ said Barbara slowly as she tried to sort out into some sort of sense the crazy thoughts which were whirring around in her head. ‘But the clock is the most important of all—it made us aware of time.’
    ‘By taking time away from us?’ asked Susan excitedly, remembering her grandfather’s words and strangely intrigued by Barbara’s theory.
    The schoolteacher nodded. ‘And it replaced time by the regular flashing light on the Fault Locator...’
    ‘Yes, it did...’ said Ian, slowly beginning to see what Barbara was getting at. He felt a thrill of excitement down his spine.
    ‘It?
    
     It?’ snapped the Doctor irritably. ‘What do you mean? Who is giving us all these clues?’
    ‘The TARDIS?’ ventured Barbara.
    ‘My machine cannot think,’ countered the Doctor automatically.
    The truth was that the Doctor was so convinced of his own superiority he had never before even considered the matter.
    Barbara, who realised how absurd the proposition would sound to someone as logically-minded as the Doctor, tried to soften the idea. ‘But the Ship does have a built-in defence mechanism, doesn’t it?’ she asked reasonably.
    ‘Yes.’
    ‘Well, that’s where we’ve all been wrong all this time. Originally it wasn’t the TARDIS that was at fault, it was us . We’ve all been so busy accusing each other, and defending ourselves from each other, that we were ignorant of the real danger. And the TARDIS—or the defence mechanism, whichever you like to call it—has been trying to tell us so ever since!’
    The possibility fascinated Ian. ‘A machine that can observe, and think for itself... Is that feasible, Doctor?’
    ‘Think, as you or I think, Chesterton, that is certainly impossible,’ maintained the Doctor. ‘But to think as a machine... yes, that is a fascinating theory. I must admit to you that there are aspects of my machine which I still don’t yet fully understand... Yes, yes, it is possible!’
    ‘We didn’t know it but the TARDIS has, of all things, been looking after us!’ said Barbara. ‘When Ian got lost in the corridors the TARDIS guided him to the Doctor: when he was trapped in that airless room, it was the TARDIS who unlocked the door for him. It even frightened me half out of my wits in the laboratory and in doing so saved my life!’
    ‘But even if that is so, how can it help us out of our predicament?’ the Doctor asked eagerly, for the first time in his life asking someone else’s advice.
    ‘You said that the power is stored underneath the column,’ continued Barbara. ‘What would want to make it escape?’
    The Doctor shrugged. ‘I’ve been racking my brains. I simply do not know.’
    ‘Something outside?’ suggested Ian.
    ‘Possibly.’
    ‘A magnetic forcer
    ‘It would have to be a strong one to affect the TARDIS,’ said the Doctor, ‘one at least as strong as that of an entire solar system, probably even a galaxy—’
    As if in affirmation the lights of the control chamber flashed up once more, momentarily blinding them, and the same sonorous clang they had heard before resounded throughout the control room.
    ‘You see!’ cried Barbara triumphantly. ‘The TARDIS has been trying to warn us all along! The lights in Ian’s room waking him up when the Doctor was about to operate the electrified controls. His door being unlocked when he had locked it... All those blackouts we had!’
    ‘Yes! But only if we went near the control column!’ said Susan.
    ‘They could have been the result of the power escaping,’ reasoned Ian.
    ‘No, they couldn’t,’ stated the Doctor definitely. ‘If you had felt the full force of the TARDIS’s power, dear boy, you wouldn’t be here now to speak of it. So great is the power that you would have been blown to atoms