Forty years ago, on the night of April 26, 1986, the explosion of reactor number 4 at Chernobyl triggered the worst nuclear disaster in history. Concealed by the Soviet authorities, the tragedy remained unknown to the world until two days later. An incredible media blackout ensued. In France, one question dominated the media: did the radioactive cloud reach the country? Did the authorities lie, as many still believe today? Did the media try to hide the truth? Through archival footage and expert testimony, this "21h Médias" program sheds light on the unanswered questions surrounding a disaster that, forty years later, continues to fascinate and raise questions.