The idea is to set the stage in ACT I, provide a complication in ACT II and close things up at ACT III. I'll try to make this system and setting agnostic.
You'll note that I assume a specific course of action taken by the players, but that doesn't mean things will go that way. The idea is to give something with a beginning, a middle and an end. If the players go another way - improvise. Some of the best sessions I had were improvised from middle to end - I usually get the beginnings right :-)
The adventurers are approached by a wide-eyed servant. He found his master - a young noble - slumped unconscious on an ancient book of some sort. He begs the adventurers for help, as he fears his master's interest in the arcane backfired seriously this time.
Finding the noble on his workshop's floor, the adventurers revive him. He babbles something incoherent about a 'portal' opening after he opened the book, and that they must destroy the book. The noble can barely speak, slurring his words, saliva dropping from his mouth. Whatever slipped through that portal must have infected the noble with a mental illness. The adventurers burn the book, and set out to find the slay the creature, later found to be hiding in the wine-cellar, digesting after feeding on the noble's intellect. A shout is then heard. It seems that the portal is still open, and something just slipped through it and out of the mansion. The noble promises half of everything he owns if the adventures make this nightmare go away quietly, without the authorities noticing.
The adventurers jolt out of the mansion, chasing the creature in the city's streets and alleys. They are then ambushed by severals thugs. After taking them down, the adventurers learn that a local wizard paid them to make sure 'no one gets out of the mansion alive'. The adventurers track the creature to the wizard's mansion. Confronting the evil man, they learn that the book was a trap - a way to get rid of a competitor. The adventurers force the wizard to close the portal and revet the creature's effect on the noble.
|(Source: Wizards of the Coast)|