The ship's first officer, Karel Kip, and his brother, engineer Petr Kip, are convicted as mutineers for the mysterious murder of the captain of the ship James Cook. However, thanks to the intervention of the governor, they only receive life imprisonment in a penal colony. There, they build a hot air balloon with which they escape. The brothers remember that a phonograph was left on the ship, which could be used to identify the captain's real killer...