Temporary maintenance worker Lu Yanchuan was once the core designer of the rail safety system. Suspected in an old accident three years ago, he concealed his identity to work as a grassroots maintenance worker. During a nighttime joint debugging session, a major hazard occurred, and he risked his life to stop the train and save everyone. In the process of investigating the hazard, he gradually uncovered the hidden clues of that year's accident. Lu Yanchuan joined forces with project manager Su Yingning to resist the layer-by-layer obstruction of capital forces, relying on on-site evidence and regulations to restore the full truth and protect the safety of rail transit and ordinary people.