On the morning of the fourth day on the Uninhabited Island, the rain drumming on the Tin Roof stopped just before dawn. Kanzaki Toru pushed open the door of the tin shack, and before him stretched a sky washed clear by the passing storm. The sea lay calm as blue-gray silk, the lighthouse's white beam had just gone out in the distance, and in its place, the first golden seam appeared on the eastern horizon. The spring on the back slope had naturally settled overnight and run clear again. The drainage ditch Ishizaki had dug carried away the last of the muddy water, and the test strips at the Water Intake showed that every measurement had returned to its pre-storm level. The solar still's windbreak ropes remained taut, each one firmly secured to the stakes driven in yesterday.
Morishita Ran was up first, as usual. She was already sitting on the edge of her camp cot, organizing the communications log. On the screen was a satellite message sent by the school at 4:07 a.m. Its title consisted of just four words: "Final Task." She projected the message onto the outside wall of the tin shack, and all six group members gathered in the morning light to read it word by word.
"First-stage final task: Collaborative Rescue Drill. Each group must complete three assigned tasks today. First: review and verify the terrain survey and optimize emergency evacuation routes. Based on actual changes to the terrain after the storm, update the correspondence between the paper map and electronic coordinates, and plan at least two new backup evacuation routes. Second: simulated casualty transport. Each group must make at least one stretcher and transport a simulated casualty from the designated coordinates to the camp medical station. No electronic devices may be used for navigation at any point; only a paper map and compass are permitted. Third: collaborative task defense. All six group members must submit a complete collaboration summary report in the evening and deliver an oral defense before the review panel joins remotely. During the defense, every member must speak at least once, covering task assignments, the rationale behind resource allocation, and reflections on the collaboration."
There was also a line of red text in the notes section: "This defense will be included in the final assessment of the Old System Shared Pool. Each group's Shared Pool weighting will be adjusted after a final review based on its performance. The final results will directly affect the initial point allocation for the second-stage class competition."
After reading the message, Kanzaki Toru turned to face the group. The morning light shone from behind him, casting his silhouette across the rust-speckled wall of the tin shack.
"Nomura will lead the terrain survey review, with Hamaguchi helping mark the new gullies and fallen trees. Ishizaki will clear obstacles along the survey route. For the simulated casualty transport, Morishita will play the casualty. Ishizaki and Hamaguchi will make the stretcher and carry her, while Nomura navigates with a compass and paper map. Shiina will record the transport time and decision points along the way. I'll handle the overall structure of the collaboration report and the defense presentation, but each of you will write your own remarks. The Shared Pool final review requires every member to speak at least once, and each person's reflections must come from them."
No one asked any questions. Ishizaki picked up the folding shovel from the corner. Damp soil from yesterday's drainage work still clung to its blade. He wiped it off with his thumb and slung the shovel over his shoulder. Nomura spread out the paper map and circled in red the locations of three new gullies marked on the post-storm satellite weather map. Hamaguchi checked the first-aid kit's tourniquets and antiseptic wipes, confirming that all the medical supplies needed for the simulated transport were ready. Morishita locked her communications equipment in the waterproof box, keeping only the school-issued satellite phone for emergencies. Shiina copied the Shared Pool final review terms onto the front sheet of her binder, highlighting two key lines: "Every member must speak at least once" and "Final results will affect the initial point allocation for the second-stage class competition."
At seven in the morning, Ishizaki led the way with the folding shovel on his shoulder. The route to the cliffside survey point had been transformed by the storm. Yesterday, it had been a narrow path wide enough for one person to pass through; today, two uprooted shrubs completely blocked it. He used the shovel handle to pry loose their roots while Hamaguchi held down the springing branches from the other side. Without a word, they worked together to move the first shrub aside, then the second. Nomura crouched on a nearby rock, marking the width, depth, and GPS coordinates of each new gully on the paper map, along with a note suggesting a location for a drainage ditch. He looked up at Ishizaki, who was wiping mud splashed across his forehead with his sleeve. Ishizaki gave him a nod—the same kind of wordless nod that needed no explanation, just like when he'd placed sports drinks on the common supplies pile.
At nine in the morning, the terrain survey review was complete. Nomura compared the updated paper map with the electronic coordinates one by one, confirming that all six terrain changes had been marked and that two backup evacuation routes had been replanned. He pressed the confirmation key, and a brief chime sounded inside the tin shack. The survey portion of the second task in the first stage had officially passed.
Next came the simulated casualty transport. Morishita Ran lay on the homemade stretcher, a simple contraption made from two trekking poles and waterproof cloth. Ishizaki had tied three reinforcing knots along each side, all following the same method they'd used to secure the still's windbreak ropes. Hamaguchi carried the front, Ishizaki the rear, and Nomura navigated with a compass and paper map. Shiina followed beside the stretcher, recording the transport time and route choices along the way. Morishita lay still with her eyes closed, playing the unconscious casualty. But as the sea wind swept through the shrubs, she could hear Nomura quietly double-checking the next fork's markings with Hamaguchi. When they carried the stretcher over a slope of loose stones, Ishizaki deliberately slowed his pace so the front end could clear the largest boulder before he followed. He told her that they'd passed the rocky slope, as if dutifully reporting to an unconscious casualty—or as if telling the people in Manabe's group who'd been talking about him a few weeks earlier: I'm not carrying your stretcher. I'm carrying out my task.
They used no electronic devices throughout the transport. Using the paper map, Nomura found an alternate forest trail that had been there before the storm washed it away, and they took it around the main path, which was blocked by a fallen tree. When the stretcher reached the camp medical station, Shiina stopped the timer. They were well within the school's time limit. Morishita sat up and took a picture of the notes she'd recorded throughout the transport, which were attached to the stretcher's handrail. She posted it in the group channel. "Casualty transport complete." Her voice was as steady as it had been when she read out the communications log that morning, but the folds of the stretcher's waterproof cloth still marked her knees.
At seven that evening, the folding table inside the tin shack had been wiped spotless. The solar lamp was set to its brightest, and all six members sat around the table. The remote defense had connected on their terminal screen, showing the review panel: Sakagami-sensei in the center, Chabashira-sensei on the left, and the second-year Disciplinary Committee Chairman on the right.
Sakagami-sensei took a sip of black tea, his gaze passing over the rim of his glasses and settling on the screen. Chabashira-sensei adjusted her glasses without speaking, while the Disciplinary Committee Chairman straightened the microphone on his desk and announced that the defense would begin. Kanzaki Toru had submitted the written collaboration summary on behalf of the group ten minutes earlier. Before the defense began, Sakagami-sensei had nodded once. The screen before him now displayed Group 12's Shared Pool weighting curve: a steady upward line, shown in green, with no sharp drops during either the freshwater crisis or the storm.
Kanzaki Toru spoke first. He explained the group's division of responsibilities and its approach to collaboration. When he described how activating the Shared Pool terms had affected the distribution of supplies, he quoted Shiina's note: "Insufficient reasons for an allocation will be automatically excluded by the system." Then he added, "But not a single reason in our Shared Pool was excluded by the system." At that, Sakagami-sensei's pen paused.
Hamaguchi spoke next, analyzing the group's strengths from the perspective of water quality testing. Ishizaki set down his log, then raised his eyes and admitted that at first he'd thought wilderness training was all about physical strength, until the storm taught him that digging a drainage ditch wasn't something one person could do alone. Morishita said that while counting the filter cartridges, she'd suddenly realized the Shared Pool wasn't just about distributing fresh water. It was also about everyone placing what they were best at into a common space. Nomura described how the backup evacuation route on the paper map had come about: at first, it had only been needed to brace against the storm, but later it had become a plan the whole group trusted. Shiina Hiyori spoke last. She didn't open her binder, but simply looked at the panel in the center of the screen and said, word by word, "The steady rise in the Shared Pool weighting curve can't be credited to any one person. But the curve itself says everything, so there's no need for us to explain every allocation decision one by one. If you'll allow it, I'd like the review panel to look directly at a note from today's transport log—the stretcher's progress over the rocky slope."
After the defense, Sakagami-sensei stopped the review panel from signing off and signaled for the display to change. In the final few seconds before the connection cut out, he straightened the microphone and said to the screen, "You activated not only Immunity, but also its sister clause: Intra-group Trust Weighting. It has been frozen since the school's seventh year, and today it was officially unlocked for the first time. Your initial points will be announced in the final review notice. Group 12, thank you for all your hard work these past five days."
He ended the call, and the screen shrank to a little timer, its second hand still moving silently. There was no cheering or hugging inside the tin shack, only Ishizaki leaning the folding shovel against the wall as the last bit of damp soil slowly slid from its blade, and Morishita Ran spreading open a notepad showing the group's initial points on the folding table.
Kanzaki Toru leaned back in his chair and let the moments no one had mentioned over the past five days pass through his mind one by one: Ishizaki's emergency biscuits, placed on the common supplies pile twice; Sakura covering the group's communications equipment with her camera bag during the storm; Karuizawa slipping mosquito patches into someone else's pack without a word; Hamaguchi repeatedly checking every piece of data before submitting the log; and Nomura drawing and erasing one backup route after another along the edge of the paper map. Every allocation had a real reason, and none had been excluded by the system. The note about the rocky slope that Shiina had cited at the end of the defense wasn't an excerpt from any one person's remarks; it was a summary of all their records. She'd said the curve would tell the whole story, and the hardest things for a machine to recognize in that curve were precisely these small human details that couldn't be assigned a point value. The school's system wouldn't approve or reject them, but together, they formed every condition necessary to activate Mutual Trust Weighting. Immunity could only be used once. Mutual Trust Weighting had no limit.
The lighthouse's white beam shone through a crack in the Tin Roof right on schedule, coming to rest on six individually wrapped emergency biscuits. The water in the Shared Pool made no sound as it reached the fill line.
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