Emily and Alexander travel to faraway Mexico. With their cousin Carlos they set sail to find the treasure of the San Gallente with the girl Maria and her father José. But José has an adversary, the nasty Barton, who owns a mini-submarine and steals every treasure José finds. But Carlos has built a bottle submarine. It's not perfect, but at least they use it to find the treasure. Unfortunately, they then run out of air, but a dolphin saves their lives.