Plots are broadly not copyrightable, “different plot” is less important than “different characters”.

I'm pretty sure the plot is copyrightable, otherwise you could just translate Harry Potter to a different language and change the names of the characters.

Well the general guideline is that copyright covers the *expression of an idea*, not the idea itself.

Translations are pretty much the textbook example of a derivative work in copyright.

Your jurisdiction may vary, of course, but it's pretty well established in mine (Canada) that "plot" is an idea, and can't be copyrighted, only the expression of the idea (e.g. the written novel) falls under copyright.

If I translate Harry Potter to another language and change all the names, the resulting book clearly expresses the same ideas as the original.

Yes, I agree.

But "expresses the same idea" isn't the benchmark, "ideas expressed in the same way" is the benchmark.

A translated work is "ideas expressed in the same way", a translation doesn't change that.

See e.g. this question/top answer on stackexchange, it details pretty well how plot can't be copyrighted using Harry Potter as an example: https://writing.stackexchange.com/questions/3928/on-copyrigh...