The Doctor had been waiting for over an hour when they appeared. They faded gradually into view, a few metres before him, hovering in mid-air as they had done before. Immediately, he snatched the intercom from his pocket and spoke urgently into it. 'Jo, Brigadier, do you read me?'
'What's up, Doctor?' came Jo's voice.
'Have you found them?' the Brigadier wanted to know.
'Yes, they're...' the Doctor began, but stopped as he realised that his instrument was suddenly giving off nothing but static. He slipped it back into his pocket and lowered his communicating device onto his head.
'Who are you?' he said using his telepathic abilities. 'What do you want?'
The random pulsations of the glowing particles became brighter and more frequent as he felt rather than heard a soft, honeyed voice inside his head. 'You are not of this planet,' the aliens observed, speaking as one collective consciousness.
'I know that,' he replied, irritably, 'but neither are you - and I want to know what you're doing here.'
'This planet is a threat to us.'
'A threat? You must be joking. The humans can only reach very few planets.'
'Not in that way,' came the reply. Then came silence.
The Doctor was about to press for an explanation but then he heard a whispered voice at his elbow. 'Doctor, are you alright? What's happening?' It was Jo.
'I'm alright,' he hissed. 'I'm communicating with them. Keep quiet!' Jo did as she was told. To her, it seemed nothing was happening. Without the Doctor's communicator or the strange powers of the glowing particles, she could not possible hear the conversation which was now taking place.
'In what way is Earth a threat to you?' the Doctor continued with his questioning.
'The Nations of this planet have amassed enough weapons to destroy themselves several times over.'
'So you want to destroy the planet before they use them on you?' the Time Lord asked scornfully.
'You miss the point, Doctor. Terrains do not have the capability to use these weapons against us, and will not have for a long time to come. They can only destroy themselves - and that is what we are afraid of.'
'Doctor!' cried Jo, suddenly. 'Your helmet! It's over heating!' Indeed, the communicating device was beginning to por off smoke, unable to cope with the stress of prolonging the Doctor's conversation. She tried to pull it from his head, only to find herself pushed away by an agitated Doctor.
'Not yet, Jo!' he cried, 'I'm on the verge of learning something!' Ignoring the pain from the malfunctioning device, he turned back to the aliens. 'What do you mean?' he continued with his telepathy. 'What are you frightened of? What are you doing here?'
'We are here to rid Earth of its nuclear weapons. Fear not for the planet or its people, for they shall not be damaged in the process. We shall simply bring about a new age of peace and harmony.'
'But why?' the Doctor wanted to know, now on his knees with pain but determined not to give in. 'Why are you doing this?'
'Were this planet to destroy itself, it would create havoc with the orbits of all nearby celestial bodies, including our own home world. We would be wiped out!'
'But for that to happen,' the Doctor protested, 'your planet would have to be one of the other eight in the solar system.'
'That is correct,' the aliens confirmed.
'Then which...?' The Doctor began - but the communicator chose that moment to give way completely, with the undesirable effect that the Time Lord felt as if his brain was frying. He heard Jo's screams, felt the helmet being wrenched from his head, sensed the arrival of the Brigadier, heard his voice:
'Doctor? What are they doing to you?' He forced himself to his knees, saw his companion, annihilator at the ready, realised his attentions.
'They...want to...destroy...our weapons,' he croaked, in a vain attempt to prevent the inevitable.
'That's all I need to hear!' said the Brigadier, decisively - and he squeezed the trigger.
There was an ear-splitting, piercing sound as a thin white beam shot from the device, and seemed to explode around the cosmic dust.
'No!' cried the Doctor. 'Don't!' He managed to launch himself at the Brigadier and knocked himself to the floor, too late.
The pulsations of the strange particles were becoming dimmer, less frequent, to the point where they seemed to be pulsating in and out of existence - and they were splitting, each one exploding into a thousand tiny granules and flaring briefly, before disappearing completely - and through it all, they could here something, a sort of howling, shrieking sound - the death-cry of the crystals, as they faded from oblivion. Then there was nothing.
'What happened?' asked Jo, nervously, staring at the spot where the cosmic dust had once been.
The Doctor shot the Brigadier a reproachful glance. 'Mankind,' he announced, 'has just lost possibly the best offer it will ever get.'
'But what were those things?' the Brigadier asked, fingering his weapon uncomfortably.
'I guessed that,' the Brigadier began sarcastically.
The Doctor interrupted him. 'From a planet somewhere in this solar system.'
'But I thought they were all uninhabited?' exclaimed Jo.
'Obviously not,' said the Doctor.
'Which one?' The Brigadier wanted to know.
'I didn't have time to ask,' the Doctor replied, 'but if they ever do come back to your planet, I doubt if they'll be so friendly again.'