Happosai performs moxibustion on Ranma, making him weak! Only way Ranma can regain his strength is to defeat Happosai and obtain the human body spot chart which can help reverse the strength-sapping technique.
Ranma 1/2 67: Eureka! the Desperate Move of Desperation
Ranma 1/2 68: Showdown! Can Ranma Make a Comeback?
Ranma 1/2 69: Ukyo's Skirt! The Girly-Girl Gambit
Ranma 1/2 70: Here Comes Ranma's Mom!
Ranma 1/2 71: From Ryoga With Love
Ranma 1/2 72: My Fiancé, the Cat
Ranma 1/2 73: Blow, Wind! to Be Young Is to Go Gung-Ho