At 6:14 every evening, a voice broke through the static on the school radio club's old receiver. It spoke only four words: 'Battery low. Position follows.' Then a string of numbers crackled out and the signal vanished.

The club's supervisor guessed that they were hearing a weather balloon, but Keira was not convinced. Weather balloons transmitted data automatically; they did not usually speak in a calm human voice. She copied the numbers for three nights and plotted them on a map. Each position lay farther east than the last, following the river toward the coastal wetlands. Whatever was transmitting, it was moving slowly.

On Thursday, the message changed. 'Battery critical. Position follows.' The coordinates pointed to an uninhabited reed island two kilometres offshore. Keira searched the maritime notices and found a report about a university team testing a solar-powered survey raft. It had broken its tether during a storm six days earlier. The raft recorded water quality, but its tracking page had stopped updating when the main transmitter failed.

The radio club contacted the university. At dawn, a research boat followed Keira's final coordinates and found the yellow raft wedged among the reeds. Its backup radio had been broadcasting on an old emergency frequency, unheard by the university's newer equipment but clear on the school's ageing receiver.

At the next club meeting, their supervisor offered to replace the receiver. Keira ran a hand across its scratched metal case. 'Maybe keep this one beside it,' she said. The machine was outdated, but being old was exactly why it had heard a signal everyone else had missed.