In a country besieged by hostile monsters/invading army, the players are sent to find an ancient army that has protected the country in the past and is now slumbering in mountains/cave/underdark/other plane.
Of course agents of rival nations (or if you want local lords that hope to take adventage of the situation to improved their own standtings) are actively plotting against the players.
When the players finally find & awaken the lost army, they
return to their homes to fight the invaders.
Unknown to all, certain promises were given to the old soldiers in the past, but weren’t kept over the centuries that passed (Maintaining a certain faith, keeping watch over the soldiers families & loved ones). When the old soldiers find out (before or after they deal with the invaders is up to the DM) they are furious & plan revenge.
At this point the players will have to find a way to make amends to the veterns & convince them to help. In addition the players might have to find a way to help the veterns re-integrate in that society after the battle.
As this is my first posting of a plot, I would appreciate feedbacks to the plot & if someone actually uses it in their game I would love to hear how it turned out.