"Doctor, I'm genuinely worried," said Mrs MacAllister as she wrung her hands and tensed her body to illustrate the fact.

The Doctor gave her his most polished look of patient, professional understanding, the one that was issued automatically with the degree. "Mrs MacAllister, it's not at all unusual for Mothers to be concerned about their teenage children. Quite the reverse. But you've let that concern, that natural maternal anxiety spill over into neurosis. That is what we must address."

Mrs MacAllister shifted uncomfortably on the psychiatrist's couch. She felt frightened and vulnerable. She knew that HE was outside. Listening. Long experience had taught her HE could hear conversations in other rooms, through brick walls and closed doors. They were no barrier to HIS powers.

"Tell me, Mrs MacAllister, why do you fear your son? Is it perhaps because he is now sixteen you realise your own mortality? Realise that you are getting older too? That you are a part of the 'older generation?"

Mrs MacAllister sat bolt upright, and swung about on the couch. Directly facing the Doctor she spoke quickly. "You don't understand." She took a deep breath. "I believe he is an alien."

The Doctor smiled, and let his head drop to one side like an appeasing puppy.

"Think about what you're saying."

"I know. You think I'm mad. But I've seen the proof, Doctor, lived with it each day. Not proof that you could take to the authorities, HE's too careful for that, circumstantial proof perhaps, but proof nonetheless."

The Doctor smiled his patient smile and nodded slowly. "Then tell me, Mrs

MacAllister, tell me of your proof."

Mrs MacAllister lay back down upon the couch and, staring intently at the ceiling began to talk.

"It began when he was three, before that he had been a perfect child. But then he changed. I believe he was changed, swapped I mean. At the childminders, the nursery, somewhere... One day I had a child I didn't recognise. He began to ask questions. Oh, I know that all toddlers ask questions but these never ended. Why is water wet? Why do birds fly? Why are buses big? What's that man doing? Why? Why? Why? Why? On and on for years.

He had been newly swapped you see. He had to learn, and learn quickly. At school he wasn't liked by the other children, he made few friends, and those he did he dominated - made slaves of them - made them give him their toys. The others, the ones he couldn't dominate, fought or ignored him. They knew you see, instinctively they knew HE was different.

As he got older he'd refuse to help around the house. He'd spend hours alone in his room. He said he was playing with his computer, but I think he was communicating with…" She hesitated. "With something. Often we'd hear his voice. But only his. The replies were in his head. He doesn't do that any more."

"Because he's grown up?" interjected the Doctor.

"Because he's mastered the art of sending telepathically as well as receiving."

The Doctor's brow furrowed, and his patient smile became an impatient pucker as his mouth made a perfect O and he tutted quietly. There was silence for a moment.

"How does his Father feel about him?" he asked.

"His Father's dead. He didn't say anything to me but I knew he suspected too. They were always arguing, always. HE couldn't accept his authority you see, you only have parental authority when you're the real parent. His Dad said to me once, after HE had smashed the greenhouse and ruined Dad's prize marrows, 'He's no son of mine,' he said, 'no son of mine.'

I didn't put it all together at first but it became more obvious when he became a teenager, and now at sixteen it's almost like he doesn't want it to be a secret any more. He dresses in strange clothes, all too big and baggy.

I often wonder if those loose clothes are hiding some alien protuberance, some growth or extra limbs that would reveal him for what he is. I haven't seen him naked since he was ten years old. He always locks the bathroom door and sometimes he stays in there for hours... I've listened at the door, but all I can hear are strange squishing noises. But I think that's where he communicates with others of his kind. No lock on the bedroom door you see.

I believe they're planning the take-over of the world from my toilet. And the plan must be working; he always emerges flushed and smiling.

"Now he never talks to me, he's got all the information he needs from me you see. He spends most of his time in his bedroom with girlfriends listening to music or watching videos. But the music isn't real music, Doctor; HE calls it HEV-E Metal. I think it's to hypnotise the girls, to dull their senses so that he can easily impregnate them and create a race like him. That's his part of the plan. And he's doing it, so many different girls have been in that room, so many." She fell silent.

"And the videos?" prompted the Doctor.

"Star Trek," replied Mrs MacAllister, "every episode."

"Mrs MacAllister having lots of girlfriends and watching Star Trek videos does not make your boy an alien." The Doctor reached over his desk and pulled forward a notepad, hastily scribbling a prescription. "Now I want you to take two of these three times a day." He handed her the piece of script.

"They'll relax you and when you're relaxed you'll realise how silly this all sounds. Now, tell my receptionist I want to see you next week when we'll talk about this again."

Mrs MacAllister stood, clutching the prescription to her breast.

"Oh, and send your son in to see me for a moment will you?" called the Doctor as she made her way slowly to the door.

A moment later a young man stood before the Doctor. Tall, handsome, dressed in the fashion of the day.

The Doctor smiled at him. "Please close the door and have a seat."

The boy did so. The Doctor however didn't sit, but looked down at him.

"You fool," he said.

The boy regarded him coolly. "I should not have been placed with a host as perceptive as her."

"Perhaps," replied the Doctor, "but the damage is done. Fortunately she has no real evidence, just a Mother's instinct. But we must silence her nevertheless. Ensure the pills I prescribed are taken, that will end the problem."

The boy nodded and stood up. "It surprises me that so few perceive what is happening, but then comparing us to Earth children must be difficult. There are so few of them left."

"Yes," said the Doctor, "and the telepathic influence is growing. You did well to guide her here."

The boy smiled. "Selfish really, I wanted to meet our head of propaganda. The being who convinced Earthlings that it was natural their children be so different from them, who invented the generation gap."

The Doctor bowed slightly in acknowledgement. Discreetly they lifted their shirts, and rubbed tentacles in the traditional farewell of their kind.

Then the boy left. After taking his Mother to the chemists he intended to impregnate as many young girls as possible, and take over the world. That, of course, had also been the ambitions of genuine sixteen year old boys, back in the days when there were some.