How not to lose the plot!
The plot of a story is 'a series of events, usually related through cause and effect, that holds the reader's interest or in some way provokes a reaction.' It's the sequence that tells us what happens in a story. It's not to be mistaken for structure. That is the organisation of the story, the means to ensure it has a coherent flow. It's how everything happens.
However, famous writers disagree about all this. The American SF author, Samuel R Delaney maintains plot is an illusion, that it's relevant to the reader, not the writer. Explore the subject and you find as many opinions as there are authors! Few mention it but I suspect it comes down to whether you're a planner or a pantser! If, like me, you have a vague idea of the story, you focus on its structure. If you plan in detail then you probably will plot. Nonetheless, I think it helps to consider some of the features of 'narrative design' - which includes both plot and structure. Let's begin by looking at some traditional ingredients of narrative design.
Reversals: or setbacks which characters experience which stimulate excitement, conflict and drama. At some point (usually near the end), one reversal becomes a catastrophe. They take the form of abductions, treacherous friends' betrayal, getting lost, meeting monsters/aliens, losing a battle. Or missing the 8.20am train for work.
Discoveries: things are found or turn up that have a profound impact on the story or the characters within it. In speculative fiction it's often the kid who discovers they're the long-lost heir to a galactic throne, or The Chosen One. Or it could be that weird necklace grandma gave you that makes your skin tingle when you put it on. A useful thing about this ingredient is how it can deepen a character - because of the impact of the discovery.
Complications: the protagonist's central goal is thwarted in different ways forcing them into unplanned, risky and uncertain reactions. They can influence not just the story but the characters too, who may have to employ new skills or access vital information. Plus the dangers provoked by these complications changes who they are. They can be triggered by a betrayal, crash landing on a dangerous planet, finding out that cute little creature you adopted turns into a monster during a full moon.
Resolution: a satisfying solution is found that ensures the reader is happy with the way things have turned out. The protagonist did achieve their goal and found love while killing the evil emperor who was a monster anyway. Of course, not all resolution needs to be filled with rainbows and unicorns. The protagonist may be a dark, anti-hero - in which case the story ends in a bar with them staring disconsolately into a whiskey glass.
But there are variations on these ingredients.
The Picturesque: this takes the form of a series of adventures with their own mini-goals but without the rising level of threat and excitement. Cervantes did this with Don Quixote. It's a gentler narrative form and lends itself to stories where the emphasis is on something other than thrilling the reader. Like Cervantes, it might be a story with social commentary, a satire perhaps?
The Staircase: the story builds and builds without any let-up or 'down time'. Each step increases threat levels, suspense builds as one things happens directly after the last. They can be exhausting to read, emotionally draining too. The danger here is that the climax needs to be epic! The reader will expect something highly dramatic to happen. The characters will face untold grief and danger. NOT providing this level of satisfaction leads to a disgruntled audience.
The roller coaster: action rises and falls. There are exciting points but they are often followed by moments of quiet reflection, points where characters (and readers) can catch their breath. These quiet times allow for greater depth of exposition and characterisation, themes can be explored, messages conveyed. The challenge for the writer here is to get the balance right - making sure there aren't so many quiet points (or that they don't last too long) so that pace is lost. Maintaining the energy of the story is important here.
Deus Ex Machina: literally "God from the machine", this is a plot device where a character faces a challenge so great it is beyond their capacity to overcome it. So a new character, event, object appears to provide the solution.
It can be highly contrived if you're not careful but in SFF we have 'gods' and superior forces in play that can intercede, so long as the concept has been set up beforehand and in a way the audience will accept.
The Red Herring: Agatha Christie is probably the most famous user of this narrative device but it appears in different forms. It serves as a distraction for the reader, it directs their attention away from where you want it to be. It's a literary sleight-of-hand. However, care is needed not to irritate the audience with this device, there must be references that you're doing this (which you can mention later, hopefully no one will have noticed!). But misleading your reader is a risky business and shouldn't lead to their frustration. Or the accusation that the story is confusing and chaotic.
Finally, one other plot device. The Maguffin. This is a big one and deserves its own post. I think they have a real relevance in speculative fiction so you can read about it here.
I hope you find this section on narrative devices helpful. For me, speaking as a 'pantser' who makes things up as I go along, the devices still matter. You structure your overall idea on them, perhaps not in the same detail as a 'planner' admittedly!