An Italian painter, Antonio Berti, pays a visit to Reims to his friend Robert Maurisson, a banker with political views on the town hall, who hides, under an affable air and an irreproachable dignity, a libertine and cynical temperament. Charged by the notable with the restoration of paintings exhibited in the cathedral, the artist calls on Cathy, a young girl from the choir, to serve as his model. The young girl will be found dead a few hours after their interview, near one of Robert’s properties. Il testimone's original motion picture soundtrack was composed by Piero Piccioni.