The mice have been invited by their cousin Pedro to attend the grand opening of the Panama Canal. He introduces them to his new friend Willy, the nephew of General Goethals who is in charge of construction. Willy and Pedro are worried that the canal will be finished in time. Frightened workers keep quitting. They keep hearing scary noises in the night and their valuables are being stolen. Pedro is convinced the scary noises are coming from El Mar Dragon- a sea monster that does not want the mighty oceans to be joined. Alexander doubts that, but agrees there is a mystery to solve. When they discover rat tracks, the mice set a trap and net the biggest rat of them all: Notail Nogoodnik. He's been robbing the workers, but he's just as frightened of El Mar Dragon as anyone. The mice must uncover the real El Mar Dragon and save Pedro from Notail's clutches before the Canal opens and lets in zillions of gallons of water!