HOMM3 picks up where MM6 ends. Queen Catherine learned of her father, King Gryphonheart, has been killed and the kingdom of Erathia had been thrown into disarray. Suspecting foul play, and the great potential for neighboring aggressors to take advantage of the situation, she leaves Enroth for Erathia with a fleet of warships. When she arrives in Erathia, her suspicions are confirmed. She finds a war ensuing, with warlords and evil barons coming out of the woodwork seeking to redraw the territorial boundaries of the land. Battling to Erathia's capitol city of Steadwick, Catherine sets herself up as Queen and beginsrestoring the land's original borders. Of chief concern are the Necromancers, whom Catherine suspects are behind the murder of her father's death.
The Necromancers raise large hordes of undead and advance unexpectedly towards the Erathian capitol. Queen Catherine enlists the aid of her father's generals and embarks on a crusade to reclaim her lost land and uncover the truth about her father's death. Along the way, other kingdoms will test her and see how much harassment they can get away with. The Elven Kingdoms close to her own will also prove stubborn neighbors, and Queen Catherine will need to find some way to gain their allegiance.
Thus begins the story of Heroes of Might and Magic 3. As the commander of Queen Catherine's forces, you must do her bidding and battling her enemies, reclaiming the lost lands of Erathia, quelling unruly neighbors, and battling the Necromancer's armies. Later on, several twists and turns will worm their way into the plot, and the story will grow even more complex. In the beginning though, you know simply that Erathia must be restored to its former glory and that Queen Catherine must avenge her father's death.