Yashimura was a criminal who stowed away aboard the sailboat Yahlen after robbing a bank. He planned to use it as a hideout until the police stopped looking for him. But his plans where ruined when Ryota and his friends Ichino and Nita boarded the Yahlen thinking it was the prize for a dance contest. Pretending to be the owner so as not to arouse suspicion, Yashimura let the trio stay on board for the night, which proved to be a mistake.
He, Ichino and Nita awoke to find that Ryota had broken Yashimura's rifle and taken the Yahlen out to sea to hunt for his missing brother Yata, effectively kidnapping the trio. Initially Yashimura was understandably infuriated. Especially when Ebirah sank the ship, stranding the four of them on Letchi Island whereupon he lost all of his stolen money in the ocean. However upon discovering that the Red Bamboo was using Letchi as a base and kidnapping the natives from nearby Infant Island and enslaving them, Yashimura's good side showed itself as he became moraly outraged at what Red Bamboo was doing and together with the others, including the escaped slave girl Daiyo, awoke Godzilla to deal with both Red Bamboo and Ebirah.
Yashimura thereafter escaped from Letchi with the help of Mothra along with his friends and the rescued slaves, before a reactor overload set in motion by a Red Bamboo scientist destroyed the island for good. He vowed to give up his life of crime.