The Ultimate Weakness Moxibustion Arc is the 28th story arc covered in the Ranma ½ manga, spanning twelve chapters. It was included in volume 13 of the Weekly Shōnen Sunday tankōbon and volumes 11 and 12 of the Viz Media tankōbon.
Ranma stops another of Happosai's panty raids and Happosai pays him back by placing a moxibustion that makes him as weak as a baby. He gets beaten up by Kuno and soon after Happosai spreads the news of Ranma's weakness. Many of his rivals including Gosunkugi take this opportunity to give Ranma a beating until he is saved by Ryoga, who showed abhorrence towards hitting those who are weaker. Ukyo offers to take care of him while Akane and Dr. Tofu try to figure out the cure.
Ranma manages to find the chart containing the cure to his weakness in the trash Happosai threw away, but Happosai quickly takes it back. The next morning, the Tendo family wake up to discover that Ranma and Genma left without a word. Akane finds out that they, along with Ukyo, went to the Yomogi Valley to train. In the valley, Genma's strategy of begging for forgiveness does not sit well with Ranma. Fortunately, Cologne shows up to properly train him. She shows him the powerful Hiryū Shōten Ha and Ranma promises to learn the technique in just three days. Cologne explains the technique in great detail and Genma, Akane (who just arrived) and Ukyo attempt to help him. However, Ranma is unable to pace himself properly to use the technique. It isn't until Ryoga shows up that Ranma is able to learn the technique. Ryoga is initially hesitant at using his full strength against someone as weak as Ranma, but he later sees Akane crying (due to her own mishap) and assumes Ranma was the cause. Realizing what Ryoga may be thinking, Ranma further insinuates that he tried to assault Akane, which adds to Ryoga's fury. With Ryoga's anger, Ranma successfully performs the Hiryū Shōten Ha. When Ranma goes to thank Ryoga, he beats Ranma up for deceiving him. They decide to return to the city and face Happosai, who had grown bored without Ranma.
Happosai quickly realizes that Ranma is trying to use Hiryū Shōten Ha, as he remembers Cologne doing the same thing when they were younger. Aware of his new technique, Happosai tries to be as pacifist as possible to prevent Ranma from executing the move. They meet at Furinkan High School and this time, Ranma uses risqué pictures of himself wearing lingerie and Happosai is finally willing to fight him. Kuno, the Principal and Mousse also join the fray. Ranma unleashes the Hiryū Shōten Ha on them, but notice that Akane got caught up in the blast as well. While everyone else is hurled out of the blast, Ranma and Akane struggle to get close. She hands him the moxibustion chart that contains the cure, but as she does so it gets shredded. The students watching scramble to gather all the pieces. When they finish gathering the pieces, they notice that the most important piece is missing. The piece is at last found by Ryoga and Cologne uses it to cure Ranma who immediately sets out to take revenge on those that had picked on him when he was weak.
- Ranma becomes weak and is defeated by virtually all his rivals.
- Ranma learns his most powerful technique, Hiryū Shōten Ha.
- Ranma defeats Happosai and regains his strength.