Indy is ordered to locate and destroy a powerful German artillery gun that is mysteriously able to appear and disappear at will, leaving death and destruction in its wake. Assisting him is a colorful group of soldiers nicknamed "The Old and the Bold" because of their old age and reckless courage. Their mission takes them on a dangerous journey across the German-held veldt via wagon train and hot-air balloon. Overcoming all manner of obstacles presented by the enemy, his own side and the harsh African terrain, Indy relentlessly follows the trail of the mega-gun right into the bowels of a secret mountain hideout, where he plans an explosive end for the phantom train of doom.