(from ebookjapan):

Kenji, Senta, and Goro were on a tramp ship bound for Brazil, South America, when a mysterious explosion caused them to be lost at sea.

The next day, the crew of the 707, a submarine of the Japanese Maritime Self-Defense Force, hears the explosion, finds the three survivors, and rescues them. The captain of the submarine says he was once a member of the same crew as Kenji and Senta's father...

The men, who met by a mysterious fate, boldly take on the many mysterious incidents that occur in the Pacific Ocean--!