After intensive training on a mysterious island called the watermelon island, Kunô sends a challenge letter to Ranma. The problem is that Kunô loses his memory following a watermelon accident. Meanwhile, Ukyô, the specialist of the okonomiyaki, decides to taste a sauce prepared ten years before. Why Ranma, Ukyô's childhood friend, is so apprehensive about this tasting? It will be the revelation of an old secret.