On a training journey in the Bayankala Mountain Range in the Qinghai Province of China, Ranma Saotome and his father Genma fell into the cursed springs at Jusenkyo. When someone falls into a cursed spring, they take the physical form of whatever drowned there hundreds or thousands of years ago whenever they come into contact with cold water. The curse will revert when exposed to hot water until their next cold water exposure. Genma fell into the Spring of the Drowned Panda while Ranma fell into the Spring of the Drowned Girl.

Upon returning to Japan, the pair settle in the dojo of Genma's old friend Soun Tendo, a fellow practitioner of Musabetsu Kakutō Ryū or "Anything-Goes" school of martial arts which Genma passed on to Ranma. Genma and Soun agreed years ago that their children would marry and carry on the Tendo Dojo. Soun has three teenaged daughters: Kasumi, Nabiki and the hot-tempered, but helpful, martial arts practicing Akane. As Akane is Ranma's age she is appointed for bridal duty by her sisters. Their reasoning is that Akane dislikes men, and that Ranma is only a man half of the time; therefore, they are perfect together. Although both initially refuse the engagement having not been consulted on the decision, they are generally treated as betrothed and end up helping or saving each other on numerous occasions. They are frequently found in each other's company and are constantly arguing in their trademark awkward love-hate manner that is a franchise focus.

Ranma goes to school with Akane at Furinkan High, where he meets his recurring opponent Tatewaki Kuno, the kendo team captain who is aggressively pursuing Akane, but who also falls in love with Ranma's female form without discovering his curse. Nerima serves as a backdrop for more martial arts mayhem with the introduction of Ranma's regular rivals, the eternally lost Ryoga Hibiki, the nearsighted Mousse, and Ranma's perverted grandmaster Happosai. His prospective paramours include the martial arts rhythmic gymnastics champion Kodachi Kuno, and his second fiancée and childhood friend Ukyo Kuonji the okonomiyaki vendor, along with the Chinese Amazon Shampoo, supported by her great-grandmother Cologne. As the series progresses, the school becomes more eccentric with the return of the Hawaii-obsessed Principal Kuno and the placement of the power-leeching alternating child/adult Hinako Ninomiya as Ranma's English teacher.