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You seem to have misunderstood. The plot is needed for the heroes' campaigns.
Specifically, for maps in Heroes II and campaigns in Heroes V. For Heroes II, the maps should be about the events that followed the War of the Succession. For example, about the love affairs of King Gryphonheart and the birth of his illegitimate children. As for the plot for Heroes V, it's still a bit unclear. But it needs to be tied to the developers' campaign.