Ryota Kane was a young Japanese fisherman whose older brother, Yata Kane, disappeared at sea. Ryota refused to give up hope that his big brother is alive, especially after their mother consulted a priestess who searched the land of the dead through prayer and could not find Yata. Obviously this meant he was still alive. Because he could not afford to buy a boat to go searching for his brother, Ryota opted to enter an endurance dance contest whose prize was a yacht. Much to Ryota's consternation, he arrived three days too late. The contest had already been going for that long and they were not admitting any more contestants.
Undaunted, Ryota approached and befriended two of the contest's losers, Ichino and Nita. Together they drove down to the docks to look at some of the boats, and snuck aboard one called the Yahlen. Once on board they encountered the rifle-toting Yashimura, who claimed to be the boat's owner. Since the trio had nowhere else to go Yashimura allowed them to spend the night aboard the ship, but during the night while the other three men were sleeping Ryota put the Yahlen out to sea.
Since no one else knew out to pilot the boat (especially not Yashimura, who not only wasn't really the Yahlen's owner but was also a criminal) the others had no choice but to allow Ryota to search for his brother: Ryota had also taken care to throw Yashimura's rifle overboard, leaving him with nothing to force him to turn back with. However it wasn't long before Ryota sailed them all into a world of trouble. A huge storm hit, and the Yahlen was sunk by the giant sea monster Ebirah, and the four men ended up washed ashore on Letchi Island.
On Letchi, Ryota and his reluctant companions discovered that the island was a base for the Red Bamboo organization, who were kidnapping the natives of Infant Island to use as slave labor. Teaming up with with an escaped slave named Daiyo, Ryota and the others broke into the Red Bamboo's base to try and free the slaves but were discovered by Captain Yamoto, the base's second-in-command. In fleeing, Ryota got himself tangled in one of the terrorists' search balloons and floated up into the air, off Letchi Island and out to sea again.
Luckily when the balloon finally deflated, Ryota was over Infant Island. He was reunited with Yata, who had been on the island since his ship had been sunk by Ebirah. The two then took an outrigger canoe and paddled back to Letchi Island with the intent of, once more, trying to free the trapped islanders. En route they were attacked by Ebirah, but saved by the sudden appearance of Godzilla, who had been discovered sleeping in a cave on Letchi and been awakened by Yashimura, Daiyo and Ichino. Once finally back on Letchi, Ryota and his brother, with the help of their friends, managed to free the slaves (and Nita, who had been captured and imprisoned by the Red Bamboo). They were all then flown off of the island by Mothra before it was destroyed when the nuclear reactor of the Red Bamboo's base overloaded and exploded.