King Gustav loves his best friend's wife. She loves him back. The innocent friend is loyal to them both at first, warning the King about plots to kill him. Over various scenes of fortune-telling, midnight herb-hunting, disguise, and a masked ball, the friend learns what's going on in a very humiliating way. He joins the assassins, and—at the ball—stabs the king (but not the wife). With his last breath, the king forgives his friend and all the would-be assassins. The final chorus declares what a lousy party that was.
One of the interesting things about this opera is that when it first opened they changed the locale to Boston from Sweden because some of the plot was based on the story of the actual assassination of the King of Sweden some 70 years before.
This made me think about a couple of scenes in a possible modern opera:
Early in the opera there is a sequence ending in a trio with Count William Jefferson, Contessa Rodham, and Thatwomeninsky - featuring the Count pleading Don't Ask, Don't Tell.
There is a later scene with a duel between Baron Algo and the Duke of Texas which needs to be settled by Justicio Antonio.