Lucie and Rio embark on a live "urbex", entering the Paris's abandoned underground network. The two teenagers share their adventure live on social media. A pair of young lawyers are on a metro platform, on their way to meet some friends. They film each other as they wait for the train. A couple say goodbye on the street. As he heads into the subway on his way to work, she goes to meet a friend for the evening. Behind her, a journalist starts doing a live broadcast. They don't yet know it, but in a few moments their fates will be bound by a terrorist attack that will change their lives. In a matter of seconds, they will enter into darkness.