When an old, unknown book was presented to the Alamire Foundation for examination in 2015, the little manuscript exceeded all expectations. It turned out to be a late 15th century songbook with a treasure trove of polyphonic music inside: 49 French chansons, preceded by the religious Ave regina caelorum by Walter Frye. The chansons are by composers including Johannes Ockeghem and Antoine Busnois, representing the cream of 15th century Franco-Flemish polyphony. In order to give this musical heritage the place it deserved in Flanders, the manuscript was purchased by the Léon Courtin-Marcelle Bouché Fund, managed by the King Baudouin Foundation, and then given to Alamire Foundation, the International Centre for the Study of Music in the Low Countries at the KU Leuven, as a long-term loan. The little songbook was christened the Leuven Chansonnier, after the place where the Alamire Foundation is located and the chansonnier is stored. The documentary offers you a unique glimpse of the history and future of the precious manuscript, the music it contains and the mysteries it entails.