The beautiful yellow Danube

Jules Verne's last novel, The Pretty Yellow Danube, was published in 1908, three years after the author's death, by his son Michel, who not only changed its title to The Danube Boatman, but also literally rewrote the work without disclosing this fact anywhere. The original manuscript was found decades later and was published by the Jules Verne Society only in 1988, and is now being published in Bulgarian for the first time.

Hungarian angler Ilija Krus decided to take a boat down the Danube from its source to the Black Sea, fishing all along the river and selling his catch in coastal towns. However, he makes the mistake of taking along a mysterious man who seems to be somehow connected to smugglers on the great river.
Who really is Mr. Yeager and will his presence derail the successful conclusion of the trip?