He was standing in front of the bathroom mirror, trying to get something out of his eye, when he first noticed it. There was no reflection in the image of his eyes in the mirror. He was looking directly at the mirror, so the image of the mirror had to be reflecting in his eyes. But in the eyes of the face in the mirror there was no such reflection - they were blank and completely clear.
At first he thought it was just a trick of the light - an effect of the angle he was at. But no. No matter where he positioned himself, no matter how he tried to move his head, the image in the mirror never contained the reflection of itself. Everything else about his image was perfectly correct - the mirror faithfully copied every move he made, when he moved his eyes from side to side the eyes in the mirror moved to. But unlike his own eyes, where the image of the mirror must, he knew, be clearly inscribed, the eyes in the mirror showed no hint of a reflection, remained a steady, patient brown.
How could that happen? he wondered. Could it be some sort of scientific phenomenon, some detail of physics that he, who had never much cared for the subject, had now forgotten. He spent an hour searching for some mention of it on the Internet but found nothing. Did it only happen to other people? He called a friend, asked her to go look in the mirror and see if she could see the reflection of the mirror in her eyes reflected in the mirror. It took a minute or two to explain this to her, and then she wanted (naturally) to know why, but he gave her some vague explanation and convinced her to go check. Yes, she could see the reflection of the mirror in her mirror eyes. Right.
Was it something to do with him then? First thing next morning he went into the men's room in office stared into his eyes in the mirror there. Yes, there it was, a faint but clearly visible double reflection. For the whole of the next week he repeated the experiment every opportunity he got. In public rest rooms, in friend's houses, in store mirrors and changing rooms - and always the reflection in his eyes was there.
So it was only the mirror in his bathroom then. How could he not have noticed it all these years? And what could it mean? The more he thought about it, the more the whole thing began to strike him as sinister. If the mirror in his bathroom wasn't an ordinary mirror, what was it? What happened to the reflection that should have been in his eyes? Did it get sucked into some kind of alternate darkness, absorbed by the mirror in some way? Was the image in the mirror even really his? Or was it some kind of counterfeit, a mock-reflection made up to look like a real one?
As the days passed the discovery, confirmed every night, came to frighten him more and more. Yet he couldn't decide what to do about it. He considered getting the mirror changed, then decided against it. If there really was some malign force at work against him, it was better not to provoke it. The important thing was not to let the mirror suspect anything. To act in front of it as though everything were entirely normal, as though he hadn't noticed the tiny flaw in an otherwise perfect impersonation. Yes, that's what he had to do. Go on using the mirror, pretend that nothing had happened.
And yet be always on his guard, ensuring that he gave nothing away.