In 1973, Dominique, a teenage girl from the town of La Machine, France, begins a dialogue with a striking spirit through the wall of her room. A year later, in 1974, after three centuries of exploitation, the last shafts of the coal mine that lies under the mining town are closed and the last miner rises to the surface.
The film interferes with these two events to shape a causal link in a bipolar investigation combining para-scientific protocols, psychological drift and group therapy. The different characters take advantage of a heartbreaking sound to travel from one geographical and temporal stratum to another. 
The deceptive silence of the inactive mine is similar to the silence of an abandoned theater or cinema. In order to disentangle the real from the fantasized, a team of scientists tries to capture in recordings the sound signatures of these cavities that one imagines to be obediently hollow. But it is a huckster rather than a guide who loses those scientists in the deep depths of the Machine.
There is this illusion which wants that a noise never comes appreciably from where it is expected but from elsewhere. In this ventriloquist's game, it is not excluded that Dominique, like Pantagruel before her, is the very theater of her obsession. As for the others participants, they would simply like to pass through the wall, to cross a threshold. Alone or in a group, they seem to want to escape the traumatic memory and tame the past. A director rather than a therapist guides them on this journey.