A 47-second birthday recording was the last sound his younger sister left behind.
Ten years later, Chen Yan, a voiceprint engineer, heard it in the media archive of a criminal syndicate.
To find her, he signed a confidential security agreement and went undercover alone, rising from quality-control inspector to a player in the operation.
Everyone said the owner of that voice was dead.
Only he knew she was still alive, somewhere in a corner of this sound factory.
