Question

How does Celebi learn Nasty Plot?

Answer 1

There was an event for the premiere of the 13th movie that gave you a Celebi that knows Nasty Plot. That is the only way (besides cheating).

Answer 2

I would assume you can use TR68 nasty plot in Pokemon sword and shield to teach nasty plot to celebi since celebi can be transferred to these games via Pokemon home.

Comments

Thsi question was posted 10 YEARS AGO

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The question never said it was asking about Gen 4, so this answer still answers the question.

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Nasty Plot was introduced in generation 4. It can't be gen 5 because, unless OP was in japan, gen 5 wouldn't have been released. Therefore, it is extremely likely that this is in fact about generation 4.

Also,  this answer can be found on the sites page for celebi, making this answer against the rules.

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Unless it says explicitly that it's for Gen 4, people might click on this question expecting answers for any generation afterward. It is absolutely fine to update old threads that people might still access today.
And it's not against the rules to share information that can be looked up easily -- that would be ridiculous. It's only against the rules to post a whole question requesting information like that.