

He had expected to find the garden on the other side. The garden of the empty house, overgrown and full of old trees. What he found instead was entirely different.
For a moment he just stood there blinking. His brain kept insisting that a wall and a garden should be on the other side of a door set into a wall, and it was taking a little time to accept that this was simply not the case.
He was standing on a hillside. A real hillside, with grass under his feet and wind on his face and a sky above him that was the most extraordinary colour, deep gold shading into green at the edges, as though the sun were setting in three directions at once. The hill rolled away below him down to a valley where a river ran, silver and quick, between banks of dark trees. In the distance, maybe a mile away, he could see the rooftops of some kind of town or village, its buildings clustered together on a rise above the river.
Milo turned around. Behind him was the blue door, standing alone in the open air, the stone wall and Hemlock Lane nowhere to be seen. Just the door, by itself, on the hillside. He tried the handle from this side. It turned. He opened it a crack and could see the lane through it, ordinary and grey and Tuesday morning. He let it close again.
"Good," he thought. He could go back.
He turned to face the valley.
The grass on the hillside was a slightly different shade of green from grass at home, a bit brighter and thicker, and when the wind blew through it it made a sound almost like very quiet music. Small creatures moved in it, not quite rabbits and not quite squirrels, with large ears and quick movements that made them hard to look at directly.
Milo started walking down the hill toward the village.
About halfway down he stopped because the path he had been following, a narrow dirt track through the grass, ran alongside a large flat rock, and sitting on the rock was a girl.
She was about his age, with very dark hair cut short and a coat that was covered in small pockets, most of which seemed to have things in them. She was looking at him with an expression that was not unfriendly but was extremely interested.
"You came through the door," she said. It was not a question.
"Yes," Milo said. He paused. "Do you know about the door?"
"Everybody here knows about the door," the girl said. "We just never know when someone is going to come through it." She slid off the rock and held out her hand. ""My name is Senna.""
"Milo," he said, shaking it. "Where is here, exactly?"
Senna looked at him the way people look at someone who has asked a question with a very long answer.
"That," she said, "is going to take a bit of explaining. Come on." She started walking down the hill. "I will tell you on the way."
The world through the door was called the Inbetween, Senna explained as they walked. It was not quite another world and not quite the same world, but something in between the two, which was where its name came from. It had been here, as far as anyone knew, for as long as the human world had been there, running alongside it like a second river following the same course.
People came through the door sometimes. Not many and not often, but enough that the village at the bottom of the hill had a small guesthouse specifically for visitors from the other side. The door moved around. Sometimes it appeared on a wall, sometimes on a cliff face, once apparently in the middle of a forest with no wall at all. It always looked the same. Small, blue, warm handle.
"Why does it appear?" Milo asked. "What decides where it goes?"
Senna shrugged. "That is the question," she said. "Nobody really knows. Some people think it looks for someone specific. Someone who needs to see something, or do something, or find something out." She glanced at him sideways. "Do you need to find anything out?"
Milo thought about this as they walked. "Not that I know of," he said.
Senna nodded. "That is what they all say at first," she said.