Plot means How the story is told.
Take Citizen Kane, because it's easy to demonstrate with. The 'story' is a reporter is assigned to ascertain Charles Foster Kane's (Orson Welles) dying words -- Rosebud. The reporter's investigation gradually reveals the fascinating portrait of a complex man who rose from obscurity to staggering heights.
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The Plot, is how the reporter goes about this in the story -- by going from one person who knew Kane to another, and interviewing them. It is through this device that the portrait of Kane is revealed. Get it? He could have gone to the library, and searched through Kane's notes and papers -- like what was done with Girl With A Dragon Tattoo. Sure, they interviewed as well, but the Plot is more on Lisbeth's hacking and research ability -- there is also the Juxtaposition of that against the older investigative journalist's methods. Or a number of other methods. What they did with Kane, however, let you tag along and get mixed up in currents that looked like still waters just a moment ago as people emptied their lives.
You get brainstorm your plot idea at first. You get the idea of how you are going to tell this story, and then you start telling it. Building it. After a few pages, you play the IF THIS, THEN THAT extrapolation game. If she is broke, then she she is hungry. If she is hungry then she is cold at night. If she is cold at night then she is open to being a criminal...
Now, where the gold is hidden and what you want to look for is any time you can change a THEN into a BUT...
IF she is cold at night, and hungry, THEN she is open to being a criminal, BUT she finds a man's wallet with $300.00, and THEN she finds a dead man who has no wallet. IF she calls the police, THEN they are going to take the wallet, BUT two men come back to the alley before she can, and take away the body.
And you keep going, adding and moving forward. Brandon Sanderson has some great lectures from the class he teaches on Fiction Writing at BYU on YouTube. In fact, he has his entire lecture series for the class up there. Here is one that is on plots and creating them -- his views are different than mine, so check him out, see what works for you...
I think there are as many ways to develop plots as there are plots. And anyone who tells you there are only 15 or 7 or 21 are missing the point of PLOT. I might agree that there are 15 or 21 'storylines'. i.e. guy saves girl from bad guy or girl saves bad guy from good guy or girl gets fed up and decides to have a CEO career instead. etc.
