Story: 
 
Saotome Ranma, son of Saotome Genma, is dragged off by his father on a martial arts training expedition to China, where they both become the unfortunate victims of a certain set of magical hot springs (Jyusenkyo). There are over a hundred springs at this site, each of which is cursed. The curse is that anyone that falls into a spring takes on the body of the first creature to drown in that spring --- but only whenever s/he is splashed with cold water. Thus, Ranma, who fell into the Spring of the Young Girl (where a young girl drowned long ago), turns into a girl when splashed with COLD water.
His father turns into a panda. The only way to undo the transformation is to get splashed with HOT water. Of course, Ranma and his father aren't the only victims of these cursed  springs...

For detail of story, check out this link:
        http://www.mit.edu:8001//people/rei/MANGA/Ranma.html