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Idea
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Setting: ???

A group of students, 2/3 guys & 1/3 gals, gang up on Nabiki to show
her what they think of her and her mercenary ways. After trying to
reasoning and threatening fails, Nabiki tries to use what little
martial arts she remembers to protect herself. Unfortunately, it's
been too long, plus Nabiki never took the Art serious, since Kasumi
was the original Heir, and after their mother died, the "title" of
Heir defaulted to Akane, the only one not trained by Mrs Tendo.

Nabiki's martial arts are weak and only anger the group. The martial
artists of the group toy with her and take some joy out of ripping her
clothes off her body. When she's down to just a torn panties, one of
the boys suggest that they rape her before they kill her. One of the
girls point out, that they would be just be wasting their time since
Nabiki doesn't "have much", Akane is better looking, even the girls in
the group are better looking than the ice queen... They tell her that
Kasumi got the brains, Akane got the brawns and Nabiki got
nothing...

As one of girls are holding Nabiki so the others can have their fun
with her Ranma arrives. He's upset at what the others are trying to do
to Nabiki. The group feels that Ranma should be helping them because
of all the dirty things Nabiki has done to him. Ranma disagrees. One
of the girls try to shock him with a picture of a completely nude male
Ranma, back-view... Ranma admits that he is aware of all of Nabiki's
"enterprises"... even those Nabiki thought only she knew of...

Ranma explains that he permits Nabiki money-making schemes involving
him since most of the money goes to supporting her family. The group
isn't interested in why Nabiki does what she does, only in making her
pay...
If this is set after Hinako-sensei is introduced, Ranma can drain the
group's battle auras. If not, he can tire them out.

Either way, Ranma takes Nabiki to a save place, where Nabiki breaks
down. Most of the stuff that the group had said were true and she is
very insecure in how she compares with Kasumi and Akane. She tells
Ranma that she is a bad person, and she needs to be punish... and
since most of what she's done was to Ranma, he should punish her...
Ranma tries to get out of it since (a) Nabiki is nude, and (b) he
doesn't know anything about punishing anyone...

So Nabiki convinces Ranma to spank her...