Ranma and his parents go to an island in the East China Sea and wind up coming back with a twin sister for Ranma, Ranko!
Ranma, Akane, Nodoka and Genma has gone for a little holiday to Jookoji-shima a little island, at the suggestion of Happosai. Ranma has a little accident, gets scraped up a bit, then falls into a mud spring. No harm done.. until that is, a copy of Ranma-chan comes out of the spring. Apparently the spring harbors the souls of many unwanted female children, and one has now been given the chance to live again. Welcome, Ranko.