Taking some examples from Feng Shui, a D&D Adventure should be easy.
THE UNDERSEA CITY
This is an adventure that shows how easy an adventure actually is to write.
Premise: The PCs are on a boat from the land of Killyronde to Caithness; however something goes awry and they are sucked down a whirlpool.
The Twist: The whirlpool is manmade, the PCs are the guests of Atlantis.
The Climax: The Evil of Dark Rapture shows himself and the PCs must repel Dark Rapture to save Atlantis and to learn a clue to the whereabouts of Spaceship America.
This Adventure can support the usual suspects of PCs. A wizard, a Cleric, some fighters, and a rogue/thief might come in handy.
In the Ancient Time there were several Spaceships that came from Earth fleeing something terrible. Takeover by hostile forces or Nuclear War. The Spaceships that traveled to Galatea to colonize the planet were the Bretonia, the Kusari, the America, the Ys, the Tartessos, and the Rhineland. The Rhineland, the Bretonia, the Ys, and the Tartessos landed safely on the Continent of Hyperia and the micro-continent of the Caithness Isles, and spread European Culture by conquering and subplanting the cultures of the Native Peoples. The Kusari landed eastward of the land of Hyperia. As for the America, no one knew where the spaceship America had landed. Everyone assumed that it had landed on the Continent of Hesperia . . . but now Hesperia is lost.
It's thousands of years later, and the High Technology that the Europeans had brought with them was lost. After two Dark Ages, the nations of Hyperia has built themselves back up into a Medieval Culture. Little do they know that there are cultures undersea, beyond the orcs, the Amazons of Amazonia, the Mystics of Skara Brae, and even the whole archipelago of Caithness itself. This civilization is Atlantis.
Atlantis itself is built by the descendants of the colonists who were on Spaceship America. They had isolated themselves from Hyperia and built undersea colonies to monitor the Hyperians and the people of Caithness. In order to work efficiently, the Americans mastered the science of Genetics and created humans capable of breathing underwater, they also created Dolphin/Human hybrids (basically anthro-dolphins). Then they built their undersea colonies: Atlantis, Poseidon, Cleitos, Tartessos, Tritoneus, and Neptune along the Hyperian Continental Shelf in shallow water.
However, during the Second Dark Age, the undersea colonies lost their connection with their Masters. The Dark Age was a terrible Dark Age. And the Undersea Colonies were forced to unify and become a nation unto themselves. Especially with enemies like the Sahaugin and other sapient, undersea life. The government is Autocratic, and the current Autocrat is an airbreather known as Lycurgus. Lycurgus rules over a nation of airbreathing atlanteans, waterbreathing atlanteans, and anthro-dolphins. Lately, the incursions of Procuros: also known as Dark Rapture, has caused the Atlanteans some concern. Also, paranoid concerns over the Surface World has caused the Atlanteans to capture "land men" to see if the Surfacers pose a threat to them.
Have your players create characters, and tell them that they should have a reason to be on the boat heading from Killyronde to Caithness. Player characters may create air breathing Atlanteans, water breathing Atlanteans, or anthro-dolphins if they prefer. Although playing an Anthro-dolphin may cause some concern among the NPCs and should be discouraged. Some suggested backgrounds for Atlanteans.
The Atlantean might be a spy or investigating the Land Men or Surface dwellers to see if they bear the Undersea Kingdom of Atlantis any ill will. So a good class for a Player Atlantean would either be thief if she is a common Atlantean, or a scholar (re: wizard) if educated as a High Caste.
Alternatively, the Atlantean could be an inventor (re: scientist) if he or she is educated as a middle class and has a curiosity of the Surface world. He could also be friends with the captain and have a machine or two on the ship (GM's discretion: I suggest that this is a submarine of some kind).
Anthro-Dolphins default to scientist, but they could be fighters, wizards, or clerics. Being underhanded and using underhanded techniques aren't really alien to Dolphins, so Anthro-Dolphins could be thieves or rogues as well.
Other player characters could include:
-- An Orc of the Horde out to see if he could make contact with one of the more moderate Orc Tribes in Caithness that did not join the Horde. He could be a fighter or a shaman (cleric or druid) or a warlock (a wizard that trafficks with, summons, or enslaves demons to his will and uses Fel Magic).
-- One of the Man at Arms hired by the captain to guard the cargo.
-- An Elven Wizard (or Elven Adventurer if you desire) who is a diplomat ready to report on his contact with the wild elves of Killyronde.
-- A halfling thief or (Halfling Adventurer) who travels with humans because he happened to be in the wrong tavern at the wrong time.
-- A Dwarf who wants to see his homeland. He can be a fighter, cleric, thief, wizard, or Dwarven Adventurer if you so desire.
-- An amazon Barbarian (Fighter) who grew up in Killyronde but wants to see new places.
Next: The EVIL BAD GUYS
Or, Sahaugin and the mutant human being that leads them.