On topic, you probably want to check out a book called Eureka: 501 Adventure Plots to Inspire Game Masters by engine publishing. It's by the guys who run Gnome Stew. Seriously awesome book.
While attending the duke's annual ball, a group of red cloaked people attack the ball. They target the duke and his family. [The enemies can have poison or magic as suits you. The red cloaks use short swords that look like
flame-bladed swords and main-gauches. Have lots of cover to use, crowds to get in the way, chandeliers, tapestries to leap on or pull down on enemies, tables to flip and climb, and sconces to try to pry and hurl at enemies.
After searching the enemies, the PCs find they all have the same tattoos under their right eyes and similar tattoos running down their right arms. Each red cloak has a small sac of gold coins struck with the seal of the neighboring kingdom. If the PCs look where the red cloaks entered they see another red cloaked figure fleeing on the roof. [leads to rooftop chase] If the PCs take time to ensure that the duke and his family are okay, the duke recognizes a symbol on the bags as the mark of a cult. [The captain of the guard will take the PCs to the safehouse that was recently busted by the guard]
Chase- the rooftop chase allows the PCs to capture and interrogate the red cloak to learn the neighboring baron hired them to assassinate the duke and his family. He also discloses when he was to meet with the baron to inform him of the assassination's success.
Safe house- investigation leads to finding plans to attack and the baron's payment. The PCs can also find plans to meet the baron after the mission.
From here the PCs can ambush the baron and learn why he was attacking the duke. Perhaps the emperor is maneuvering behind this all, or the baron just wants the duke's holdings.
Have you ever seen the train job episode of Firefly? Watch it. Steal basically all of it!