At the turn of the millennium, Chen Yan was a rookie safety officer at a chemical plant. When the ammonium nitrate production line overheated, he failed to sound the alarm, resulting in a catastrophic accident with heavy casualties. He took full responsibility and died in despair. Reborn, he returns to the moment before the disaster and decisively sounds the alarm, organizing an evacuation that saves all more than 3,000 people without a single injury. Han Shouye and his accomplices are brought to justice, and Chen Yan is finally cleared of blame. Five months later, Chen Yan returns to his post, vowing never to let anyone cross the red line of safety.