Anyway there's something that I don't understand. I have some screen-shot so I hope I will be more clear about my problem:
View attachment 44954
Here I have enough honour to attack (an attack on Sagara clan would cost me only 14 honour)
View attachment 44955
Here it is the "strange thing": it tells me that target of the plot is Ashikaga, me :huh:
I've tried to buy honour with Emperor gift but in any case I never had the "plot attack button" available.
I think there's a bug with the translation here, our localization team has mixed up the names. It should say that your character has to be at war with the plot target.
Maybe I have to attack first as a clan and only then unleash plot fury?
Yes, you have to start the war in usual fashion and only then can you trigger the plot.