The Plot of an adventure provides the creative skeleton around which all the action takes place. Without a plot, the One Shot Adventure simply becomes a series of unrelated scenes, and it is very difficult to get player buy-in and enjoyment from such. This is also a symptom of a weak plot, which simply goes to show that having a bad plot is just as bad as having no plot at all. My goal as a Game Master is simple: create a fun and compelling gaming session for all those involved. The first step in that process is have a good plot.
Good plots, as stated in the first article in the One Shot Adventure Creation series, comes in the form of a single sentence that captures the task the adventurers must pursue, the target of that task, and the opposition that creates the challenge. You can get ideas from movies or TV shows you like to watch, books you like to read, other campaigns you've played in, and other obscure resources (like what's going on with that talkative cubemate in the office, or tales lifted from ancient mythology). If that works well for you, great! Sometimes, though, some of us need a jump start.
When I am looking for inspiration and I just can't find it, or when I want to challenge myself, I turn to Random Charts and Tables. Those are a lot of fun for me, because I have to either find a way to make sense of the results I get, or I find inspiration as I go along and that becomes the seed I use to develop the actual plot. It helps to develop your own charts for this, but I can show you an example of the kind of thing I use.
For the inspiration for my main plot chart, I turn to The 36 Dramatic Situations by Ploti, based on an RPG article I read that introduced the concept to me. While not entirely comprehensive, the 36 Plots works very well for creating a wide range of stories. We all know stories that follow these basic plots, and we can easily turn those into the source of our inspiration for an adventure. Besides, 36 is a a great number for a D66 table (from Traveller, wherein you role 2d6 in a manner similar to percentile dice, where the first die being your tens place and the second die being your ones place, to create 36 different results.)
Alternately, I can create a simple list of goals for an adventure, such as Steal Item, Explore Ruins, Save Princess, etc. and turn that into a quick Plot Task generator. The advantage here is that you can create more focused and specific plots very quickly using that as your inspiration. The 36 Plots method is very generic and can admittedly take more effort to create in terms of a specific plot, but given a wider range of plot choices, I get a wider range of adventuring possibilities as a result. However you create your Random Plot Task generator, feel free to choose the approach that seems to inspire you the best. That's what it's there for.
I will follow up on this article with some random charts later on today, to provide some examples of what I'm talking about here. For now, though, think about what kind of plots you like to run for your adventures, or the kind of plots you like to play in when you are on the other side of the GM Screen. That's the best source of plot ideas you've got, right there.