Marine Boy enters a kind of submarine Grand Prix using a special boat invented by Dr. Mariner and Professor Fumble, but the boat is blown up before the race is finished. Marine Boy is knocked unconscious in the explosion and, although he recover briefly when given oxygum, Marine Boy soon faints and is taken to the hospital. While in the hospital, an enemy agent drugs Marine Boy's medicine but he realizes it and escapes. Marine Boy returns to the crash site and follows a frogman into a cave. However, it is a trap and the cave fills with knockout gas, causing Marine Boy to fall asleep. Can Neptina and Splasher save the unconscious Marine Boy?