Every time I sit down and prep a one-shot game, I tend towards the reliable plot structure that I’ve used, especially if it’s my usual style – a pulpy action/adventure. It might sound like this is a boring constraint, but it’s also a useful one – it can be easy to over-think structure in one-shot games.
I should note that there are plenty of games that this structure doesn’t apply to – PBTA games, for example, lend themselves to a much more loose prep style that allows the players to relax into their PCs and follow their noses much more, as I’ve suggested here.
In short, I build my plot around three parts (I’m not a fan of calling them “Acts” – it all seems a bit am-dram) –
Part 1 – is a big, actiony open. It’s probably a fight scene, but it might be a chase or a tense investigation. It should set up the situation the PCs find themselves in and where they could be heading.
Part 2 – is really a collection of options. Here it can be quite loose – a few clues from Scene 1 might lead them around different places, adding up evidence or just covering ground. These mini-scenes can be quite short, flavour scenes that reinforce the tone of the game, or simple skill checks that can be overcome and allow some peripheral characters to shine. Eventually, though, they will all lead to…
Part 3 – a big climactic fight. The PCs should be stretched by this fight – the one-shot mortality rule is that it’s OK to kill the PCs in the last fight, but before this is usually a poor show.
And that’s it. Whatever your premise, whatever your system, a tight one-shot game probably boils down to the sequence above. Quite often, when I review my own games and the plot hasn’t really sung, it’s been a reluctance to follow this 3-part structure (such as here, where I pretty much missed out Step 2).
Over-simplification? Or am I just stating the obvious? Credit where it’s due, there’s a lot more detail on this here, on The Alexandrian.