Free Doctor Who: Mawdryn Undead by Peter Grimwade

Nyssa, Tegan and the Brigadier were stranded in a no-man’s-land of uncertainty, half-believing that the mutated being was the Doctor, desperately in need of their help, and half-convinced that he was a dangerous imposter.
    Mawdryn trembled; so much was at stake. If only he could get the TARDIS to the ship. If only the genuine Time Lord would follow in pursuit. He struggled hard to remember the names he had heard the man and the two girls use to each other. ‘Tegan! Nyssa! Brigadier! My old friends!’ he pleaded. ‘Please help me!’ He leaned, exhausted, against the console. Tears of frustration flooded his eyes.
    Tegan and Nyssa looked helplessly towards the Brigadier, who had already made up his mind. If there was the remotest chance that this fellow was the Doctor, he had to be given the benefit of the doubt. Nyssa moved to the control panel.
    ‘Do not enter new co-ordinates. Activate sequential regression,’ ordered Mawdryn.
    Nyssa obeyed. ‘We’re ready to leave, Brigadier.’ She hesitated to close the doors.
    ‘I’m coming with you.’
    ‘But Brigadier...’
    ‘Don’t argue!’ He silenced the two civilians, both of whom were only too glad of his company.’
    The double doors were shut, the co-ordinate settings reconfirmed, all checks completed and Nyssa’s hand poised over the dematerialisation control, when the drone began.
    It was a sound they had never heard before.
    ‘Not another alarm?’
    ‘I don’t know,’ said Nyssa moving round the console. I think it’s from the communications system.’
    The Doctor stood up, looking rather pleased with himself; the transmitter was working. Already the signal should be reaching the TARDIS. ‘Tegan and Nyssa can use the beam as a beacon,’ he explained to the Brigadier. ‘If all goes well the TARDIS will reappear... Oh no!’ He stopped. ‘Quickly, Brigadier!’ He grasped the old soldier by the shoulders.
    ‘Think! Did you go on board the TARDIS with Tegan and Nyssa?’
    ‘I can’t remember. Does it really matter?’
    ‘Of course it matters! Can you imagine what would happen if you walked out of the TARDIS in 1977 and met yourself in 1983?’
    ‘That’s ridiculous.’
    ‘Not ridiculous, but almost certainly catastrophic.’
    ‘You mean I could be two people?’
    ‘Certainly.’ The Doctor tried to impress upon the bemused Brigadier the seriousness of the Blinovitch Limitation Effect. ‘You could exist twice over, but because you’re basically the same person any close contact would short-circuit the time differential created by the journey in the TARDIS. The energy discharge would be entirely unpredictable.’
    The Brigadier struggled hard to remember what had happened when he and Tegan reached the top of the hill in 1977. From the darkness one faint, misty image emerged; a vanishing TARDIS... he was left beside the obelisk... alone!
    He tried to explain the distant, curiously upsetting recollection to the Doctor.
    Neither of them saw Turlough dart forward to the buzzing transmitter. There was a noise like a firecracker and the Doctor swung round from his conversation with the Brigadier to see a small whisp of smoke over the apparatus. He rushed forward to inspect the damage.
    It did not take him long. ‘We won’t be going to the ship,’ he announced.
    ‘The transmitter is useless. I’ve lost all contact with the TARDIS.’
    The Doctor’s dismay was nothing to the relief of Mawdryn when the drone in the TARDIS stopped sounding; any communcation from outside would be disastrous to his plans. ‘Dematerialise!’ He commanded.
     
    Nyssa prepared to obey.
    ‘Wait!’ Tegan held back her fellow companion. ‘If that sound came from the communications system, someone might be trying to get in touch with us.’
    The girl was not as stupid as her brashness suggested.
    Mawdryn’s putrid flesh quaked.
    ‘Perhaps it was the Doctor,’ added Tegan.
    ‘ I am the Doctor!’ shrieked Mawdryn. ‘Dematerialise immediately!’
    Nyssa