At last, we can put an outline together on our scenario. Outlines are pretty important, since they give you a basic idea of how an adventure is put together. We are going to use a Step Outline for our adventure: it's a good way of outlining our adventure using the Premise, Twist, Climax formula for our adventure (all adventures have four acts, by the way).
Adventurers of Atlantis
Premise: There has been a rash of kidnappings of young women of late. Something the PCs scoff of until one of their girlfriends disappear.
Scene 1: the PCs have been hanging out at their apartment (or University Student Common building, if you are in the U.S.) and learn that there are a rash of kidnappings of young women (18-24 years of age) in their city.
Scene 2: Girlfriend shows up and tells a wierd story of how she went to a strange man's apartment and exposed herself and served him.
Scene 3: Girlfriend disappears. Investigating shows that a weird little man has been hanging around campus (or near the places of disappearance).
The Twist: A strange old man is found by the PCs, who is actually an Atlantean Theorist. The PCs chase the wierd little man, only to enter a dimensional rift through space time and end up in the city of Atlantis.
Scene 4: the PCs find the weird little man and they chase him until he reaches a bridge and he seemingly jumps off. If the PCs jump after him, they fall through a hole in Space/Time, landing in the City of Atlantis.
Scene 5: Exploring Atlantis, they find a society that is highly advanced with many pyramids around. However, they are dressed strangely, and they've been attracting a lot of attention lately.
Scene 6: One of the PCs find their girlfriend on a slave auction block, stripped nude and exposed to a lot of people. If the PCs do anything to try and save their friend, they are taken into custody for disturbing the peace.
The Climax: The PCs are told that a group of Lemurians are attacking Atlantean interests in Magan (modern day Kenya). The PCs battle it out with the Lemurians in Maganese lands.
Scene 7: For all their strangeness, the PCs are eventually released from Custody and will have all charges dropped against them if they will do something for Atlantis. Basically, stop the Lemurians from harming Atlantean interests in Magan (modern day Kenya).
Scene 8: The PCs are sent in an Atlantean jet aircraft (if the GM wants to be more fantastic, he can change it to a flying chariot or something like that). They then are teleported to the Magan coast where they meet the Lemurians.
Scene 9: the first meeting with the Lemurians did not go well, and it results in a fight. The Lemurians either slip away or defeat the PCs (and lets them live) while they continue their objective, the Maganese Uranium Depot.
Scene 10: the PCs meet the Lemurians at the Uranium Depot and they battle it out royally.
The Closing: The PCs return home, and one of the PCs is given the feeling that the Atlanteans might call upon them again.
Scene 11: After defeating the Lemurians, and gaining an enemy, the PCs return to Atlantis silently hailed as heroes. They proved themselves to the Atlantean government and are allowed to go home. Any PC who wishes to purchase their lost girl friend may do so to save her from a life of slavery.
Scene 12: After going through a hole in Space/Time again back to the modern day, the PCs return to their modern lives. Although, give the wisest a chance to sense that the Atlanteans are not through with them.
There you go, a step outline for our adventure. The adventure is completely outlined, so you can run it. Remember, not every adventure will go as planned. The first rule of playing a Roleplaying Game if you are the DM running an outlined adventure is to expect the unexpected (players do strange things like mess everything up in a planned adventure).