Outside the windows, the light went.
It happened in the way that wrong things happened — gradually and then suddenly, the way the temperature in a room dropped before you identified the reason. The grey afternoon deepened past what the clouds could account for. The carriage felt colder in a way that was not the cold of open windows or mechanical failure. It was a cold that came from somewhere less explicable than either.
"Something's wrong," Hermione said, quietly.
Ron set down the book he had been reading and looked at the window. The glass had a quality it hadn't had before — not quite frost, not quite shadow, but both of those things at a level that existed beneath the physical.
Lupin had already moved. He was on his feet in the smooth, unhurried way of someone who had trained himself not to betray the urgency that was actually present, looking toward the corridor with the focused attention of someone who had encountered this before and knew exactly what it was.
"Stay in your seats," he said. Not loudly. With the specific authority of someone who didn't raise their voice because they didn't need to.
The compartment door slid open anyway.
What filled the doorway was not quite there in the way that things in the physical world were there. It was tall and robed and the cold that came off it was not temperature but something that operated on a different system entirely — the cold of the specific conviction that nothing good had ever happened or would happen, that the warmth you remembered was something you had imagined, that the people you loved were gone or would be gone and the distance between those two things was not as significant as you had believed.
He was not unprepared for it, which was not the same as being unaffected by it.
He pressed the Occlumency down around his thoughts like a closed room and breathed and felt the cold try to find purchase on something and find the surface he had constructed and slide off it. Not entirely. Not without effort. But enough that he could observe what was happening rather than being consumed by it.
Harry made a sound.
It was not a sound Ron had heard before — low and involuntary, the sound of someone for whom the Dementor had found the specific thing it was looking for, which was different for every person and worse for some than for others. Harry's worse was a category unto itself.
Lupin was already in motion. He stepped between the Dementor and the compartment with a Patronus casting in progress — the silver light building in his wand with the practiced ease of someone for whom the first step was automatic — and Ron watched it with the focused attention he brought to everything he wanted to learn. The wand movement. The incantation. The light that followed it.
The Dementor retreated.
The cold receded by degrees, which was not the same as leaving entirely but was enough.
Harry had gone the grey-white of someone whose blood had done something involuntary, and he was no longer sitting upright. Ron caught him before he could slide off the seat — more reflex than decision, one arm across his shoulders, steadying — and Hermione had already produced chocolate from somewhere with the efficiency of someone who had read extensively about Dementor exposure and had prepared accordingly.
"Here," she said, breaking off a piece. "Eat this. It helps."
Lupin settled back into his seat with the contained quality of someone managing their own response with professional thoroughness. He reached into his battered case and produced a significantly larger bar. "Honeydukes' best," he confirmed, breaking off pieces and distributing them with the matter-of-fact generosity of someone who had decided chocolate was going to be distributed and was distributing it. "I've found the quality matters."
Harry ate the chocolate with the slightly dazed quality of someone returning from somewhere they hadn't chosen to go. Colour came back to his face by degrees.
"I heard something," Harry said, eventually. "When it came in. A voice."
Lupin looked at him with the careful attention of someone who had heard this before and was present with it rather than managing it from a distance. "Yes," he said, quietly. "That happens. Particularly for people who have experienced significant loss."
Harry was quiet. He wasn't going to say what he'd heard, which was a decision Ron respected.
"The Dementors will be searching the trains for Pettigrew," Lupin said, to the compartment in general, with the directness of someone who had decided information was more useful than the pretence that nothing had occurred. "They won't be a fixture at the school itself — Professor Dumbledore has made that clear. But they will be present at certain perimeters, Hogsmeade station among them." He paused. "Chocolate remains relevant."
Ron took a photograph of the condensation on the window — what the cold had left behind on the glass. He wasn't entirely sure why, except that the summer had taught him that the photographs worth having were not always the obvious ones.
He put the camera away and ate his chocolate and watched the English countryside gradually remember what light was supposed to look like.