Bullton and Piper find a missing fishing boat with no one aboard. Marine Boy and Splasher set out to solve the mystery of the crew's whereabouts. Marine Boy enters the fishing boat and meet a strange man, Professor Doomsday, who offers to show Marine Boy something amazing. However as he turns, Doomsday hits the young hero hard on the head and Marine Boy is knocked unconscious. When he wakes up, Marine Boy is laid across a rock. He is suddenly attacked by a giant shellfish and, although he escapes, Marine Boy faints and heads toward some rocks! Can Splasher save the unconscious Marine Boy?