A script is anathema to a RPG.1
First, no plan survives contact with the enemy. The same is true of RPG detailed plots. You can try to make your players go a certain route: this is called rail roading and is generally viewed as a bad thing™. Even if you plan all possible moves, the players will chose one you did not think of.
Second, player agency matters. This means that the players must have an impact on the world. Their (in)actions should matter. They are the heroes of your tale, not actors to either support your favourite NPC(s) or witness the brilliance of events happening around them.
Finally, knowing what type of game everyone wants to play is essential: the same page tool is a good starting point to get discussion going as to what you all wish the game to be like.
As a side note, system matters so choosing a system (Cypher, Fate, Apocalypse World, D&D, etc…) should be obvious. But that requires you know about those choices in the first place. There are a few generic system (GURPS, Cypher, Fate, …) that can be made to fit any setting (though not necessarily any type of story). Or you can use no system at all2.
A good script!
A good script can give you a good idea of what the major NPCs and their factions will do provided that the PCs do not mess with it. This is actually really useful. It allows you to work out how factions interacts, how they respond to changes of circumstances, and partition their knowledge.
A good script can serve as a good base for a lore document which can be given to the players to read. Or rather, different versions can be given to each players depending on what their character knows. It could be your "the story so far…" type of thing.
There is a fair amount of jargon in this answer (player agency, system matters, etc…) which you can search for on the wider net or on here for more detailed explanations.
1: A bold statement that require to be taken with a grain of salt. Some script can work for some games but there are mostly exceptions to the rule.
2: A somewhat controversial choice but the one I favour and have been using for years. Although I would only recommend this for experience GMs.