The forest does not want them there. Fritz can tell from the way it moves.

Every leaf turns toward the sun like a dish catching rain, and the sun pours down through the canopy in hard white columns that his visor dims to the color of weak tea. Spores drift through the light, slow and golden, pretty enough to make you forget what they do to lungs. His father made him repeat it before they left the last camp: Seal checked. Filter checked. Never touch the bark.

The three of them walk single file. His father leads with the beacon reader, his mother follows, and Fritz trails at the back, where he can be watched. Their sun-safe suits are bulky and gray and patched at the elbows, and every step makes a sound like a sleeping bag being folded. They skipped lunch to make the walk before the afternoon heat, and nobody has complained, not even Fritz. That is how he knows this one is different. In the camps, somebody always complains.

"Two more kilometers," his father says. His voice comes through the helmet radio, crackling at the edges. "Maybe less."

"You said that an hour ago," says Fritz.

"And it was true then, too."

His mother laughs, a short bright sound, and Fritz grins inside his helmet where no one can see it.

Doctor Hank Bismarck has been the most valuable man in four settlements. People have traded him batteries, seed packets, and once a whole live chicken to stitch a wound or read a fever. The family has moved from camp to camp because a doctor is always needed somewhere else, and Fritz stopped unpacking his bag years ago. Then, three weeks ago, a message arrived on his father's handset. It came with no sender, and it was encrypted so heavily that his father needed two days to open it. Fritz read it over his shoulder before someone shooed him away.

We need trained physicians. We can offer you shelter from the sun, work worth doing, and a future for your son. If you accept, you will not be able to return. Follow the beacon.

"What do you mean, not able to return?" Fritz asked that night.

"It means," his father said carefully, "that they keep to themselves."

That was all anyone would tell him. Fritz spent the three weeks inventing explanations. It was a bunker built by a government that no longer existed. It was a cult. It was a city of robots. It was a farm with real cows. By the time they set out, he had decided he did not care, as long as it had a ceiling.

The beacon reader chirps. His father raises his hand, and they all stop.

Fritz looks at the reader, not at his feet, and his boot goes straight into a loop of root that rises out of the ground like a handle. He pitches forward. The world tilts, all trunks and glare, and his arms fly out on their own and catch a passing tree.

He is holding the bark.

For a long second nobody says anything. Fritz stares at his gloved hands pressed flat against the trunk. The bark is furred with pale green moss, and where his fingers have pushed in, the moss releases a soft puff of dust that hangs in the air before his visor.

"Let go slowly," his mother says. Her voice is very calm, which is worse than shouting. "Step back. Don't breathe hard."

He lets go. He steps back. The dust drifts past his helmet and away.

"Gloves," his father says, already beside him, turning his wrists over. He checks the seams, the cuffs, the seal at the forearm. "Gloves are fine. You're fine."

"I'm fine," Fritz repeats. His heart is going very fast. "Sorry."

"Watch the ground," his father says, squeezes his shoulder, and that is the end of it. But Fritz keeps looking back at the tree as they walk on. For a moment, before he let go, he felt something through his gloves. It wasn't heat, exactly. It felt more like a hum, as if the whole forest were holding one long note.

There is danger in leaving the path, and there is a thrill in it too. He is not sure yet which one he is feeling.

A whistle sounds somewhere ahead of them, two notes, rising.

His father stops, cups a hand over the front of his helmet out of habit, though it does nothing, and whistles back through the radio's external speaker: the same two notes, rising.

They walk into a clearing. It is small and round, and the light falls into it so hard that Fritz's visor goes nearly black. At the far edge, a figure in a sun-safe suit stands with one arm raised. The suit is newer than theirs. It is white and smooth, with no patches.

The figure beckons. Fritz sees nothing behind it but underbrush until the figure bends, grips a handle hidden in the ferns, and pulls. A square of forest floor lifts up on hinges. It is a hatch, painted so carefully in greens and browns that even standing three meters away, Fritz had thought it was ground.

A woman's voice speaks inside his helmet. It is a recording. He can tell from the little hiss before it begins.

"You must be the Bismarcks. Welcome, all of you, to Paraville. We have warm meals waiting for you in the cafeteria. If you need anything while you settle in, I'm the one you can call. I'll be your intake coordinator. My name is Miranda."

"Nice to meet you, Miranda," says his father, as if she can hear him. Perhaps she can. "We're grateful to join your settlement. I'm glad to be of service."

The figure by the hatch tilts its helmet. Then a live voice comes on, the same woman's voice, warmer and a little out of breath. "Thank you for answering our call, Doctor. We've needed a physician for a long time. Mind the ladder. It's a long way down."

It is a long way down. Fritz goes third, after his mother, counting rungs until he loses count somewhere past ninety. Above him, the square of sky shrinks to a postage stamp and then to a coin. He hears Miranda climb in after him and drag the hatch closed. The coin goes out.

For a moment there is only the dark and the sound of his own breathing.

Then lights come on below him, one after another, in a long descending line. He looks down between his boots and sees the shaft open into a space so large that his eyes refuse to understand it at first. Walkways and windows and green things grow under lamps, and a broad curved ceiling is painted the soft blue of a morning he has only seen in pictures. People too, small with distance, walk without helmets and without suits, their faces bare to the air.

"Oh," says his mother quietly over the radio. That is all.

Fritz is tired and hungry, and his arms ache from the ladder, and he hangs there on the rungs and simply looks. The camps above, with their tarps and tin and rationed shade, will never compare with this. Down here, a person can stretch. Down here, a person can stay.

At the bottom of the ladder there is a little airlock room with benches and hooks for suits, and Miranda helps them out of their gear one buckle at a time. Out of her suit, she is a small woman with short gray-streaked hair and a quick, efficient smile, and she smells faintly of oranges.

"Suits go here," she says. "You won't need them again for a long while. Most people never do."

"Do people go back up?" Fritz asks before he can stop himself.

His mother gives him a look. Miranda only smiles and hangs his helmet on its hook beside the others. Fritz notices there are a great many hooks, and a great many helmets on them. Most of them wear a layer of dust.

"Not often," says Miranda. "There's nothing up there we need."

Behind them, with a sound like a long exhale, the inner door seals, and a row of small red lights above it turns, one by one, to green.

Fritz will think about that sound later, many times. In the moment, it seems only like a door. It will take him a long while to understand that it was the sound of the whole world closing.