Generating an Adventure

Creative Process at Work
I have spoken with and read about authors describing how they come up with ideas. Many of us borrow ideas or take something fairly common and give it an interesting twist. Some are inspired by dreams, others by conversation with friends. The following idea pretty much came to me as is, although it certainly isn't original. This is a Mythos adventure idea set in the 1920's. I don't have a regular CoC game so I am thinking of it in terms of a one-off with pre-gens.
The PCs are aboard a Japanese freighter out of British Columbia heading to the home islands with a cargo of luxury goods for Japan. Cargo includes a number of large hardwood logs bound for a Shinto temple, an expensive motor car complete with PC mechanic/driver, a number of prime beef on the hoof (the starting of a herd?) and a PC hired to clean their stalls, an ancient stone alter and several other large stones from a Canadian archeology site accompanied by an eccentric Asian scholar (NPC) and his batman/body guard. Throw in a lumberjack PC who is off to see the world and a New Orleans gambler PC on-the-run and I think we have a good crew. Well, add in the ship's captain and mates one who "has seen too much", and now we have a crew.
The ship setting really confines the PCs to a small environment, but I like to run things loose, so that is for the good...the PC investigators get a lot of freedom and they can't stray far. This is a "time-line" adventure so things will progress along a course of action unless the PCs do something to halt things. A few days out to sea the "accidents" start. A crewman is badly injured, machinery breaks down, etc. The spooky crewman starts talking weird, says "the cargo is bad mojo". "The trees are speaking to him" (sort of True). "The food is poison" (False). Rats seem to have taken over the ship below decks as the ship cats and rat terrier are all huddled in the crew quarters above deck. Someone leaves small offerings on the alter (dead rat, dead cat?).
Asking around the investigators will learn they are headed for Japan by a normal trade route. There should be good weather this time of year and an unusual night sky configuration not seen for centuries to make the journey memorable. However, the cattle sicken and this worries the PC who cleans up after them. Some of the drinking water becomes sour and one morning the cook is dumping spoiled food overboard...a school of sharks follow. The ship slows and PCs are told there is "engine trouble". There are strange lights and mists after dark.
If the investigators question the crew, some will seem frightened, the spooky guy will steadily get worse (talking to himself and "the voices, make them stop", he says), the captain seems to be hiding his concern, but develops a noticeable "tic". The Asian scholar will entertain guests. He is an eccentric like many scholars and studies ancient civilizations (he may go on about Egypt, etc.) and his hobby is newspaper comics. His batman is silent and very muscular (martial artist). The logs are indeed haunted and angry spirits will interact with any PC who is sensitive to that sort of thing.
The weather turns bad and the mood of the crew sours. The injured sailor dies. The spooky guy hangs himself from the yardarm in a Christ-like pose. The constellations are in order in an "unnatural" sky and the scholar is ready to perform his ritual of summoning. Things from the deep climb aboard ship and assist him in slaughtering most of the crew in a blood sacrifice. The ancient stones and alter are from a mythos worshiping cult and are be used by the scholar to call the great old one from the deep. If the investigators don't prevent this, Cthulhu will stir causing an earthquake and tsunami. He will rise and take the ship to the bottom.
The PCs will check for lost sanity for interacting with the spooky crewman, tree spirits, the deep ones and slaughter of crewmen, obviously the old one and being ship wrecked (they may grab onto a log or other debris). A few hours/days later (may depend on whether PCs got an SOS out) the survivors are picked up by a passing freighter.
A ship's plan, character sheets for the captain, scholar, body guard, spooky guy and PCs, plus monster stats is about all I need to do in terms of preparation. I would guess this could be run in a long evening or split into two shorter sessions. The plot just seemed to write itself as I thought through the situation. I am of course drawing upon all the stories, movies and other sources of sinister tales I have accumulated in my brain over almost five decades of soaking them in. Is this derivative ? Of course it is, but role-playing adventure games can take a well known story and because it places the players in a "you-are-there" situation, it's all fresh and anything can happen. Players may take really unexpected actions, such as abandon the ship well before the summoning, trusting their luck to being discovered on the well traveled route. If so, then that becomes the story. Improvisation is always required, it only remains to see how much improvisation player actions lead to.