A plot starts when something either happens to the main character, or he/she causes something to happen that throws his/her “normal” life into upheaval.
And the rest of a screenplay is about this main character doing things to overcome obstacles in order to get that normal life back on track; ending up that he either gets his/her previous “normal” life back, or he/she settles into a new “normal” lifestyle.
This basic plot is then structured in, usually, three acts.
The first act (the first 25 or so pages) sets up the main characters as well as the premise of the story. This first act should have a “hook” that ideally appears by page ten. A hook is a scene that immediately sucks the audience into the story making us want to learn more.
This first act ends with a major plot twist that propels us into the second act.
The second act (the next 50 or so pages) is then composed of the obstacles placed in the way of the main character that he/she must overcome to achieve his/her goal or solve his/her problem.
These obstacles must increase in intensity, suspense, tension, drama and/or comedy (depending on the genre of the story) until at the end of this act a crisis point is reached.
This is the make/break point for the character. It’s his/her last chance to win or lose, solve his/her problem, or attain his/her goal, or all is lost.
The third act (the last 15 to 20 pages) is then the ending where the old “normal” is restored or a new “normal” is achieved (or not, if it’s a “down” ending) and all loose ends are wrapped up.