In that regard, creating this mission required that the scene exists first, without thinking that this is going to be a mission scenario for a game.
Action/Adventure stories are arguably the easiest to convert into a game. Your characters are involved doing things, and everything hinges and focuses on these things they do and how it affects their progress through the story.
Now, since this is the first scene, it must contain the first major dramatic twist. It should present the way things are, and then an event should occur that turns things up side down, thus introducing the audience to the symptoms of the central problem of the story.
This however should be established so that it actually means something to the characters. Say, for example one event does happen that means so much to someone you are looking at. Without knowing first the things involved in that event, we will never understand how this can be important.
This will mean we could just shrug the event off, no matter how eventful it was for the character.
Turning this script into a mission is not automatic, and cannot be forced. It must come to you naturally, and it will. This is the time adjustments must be made, but this doesn't mean the tensions in the script will be broken or lessened. In fact, it can be amplified even more.
The only limiting factor will come from the absence of actual actors that will show the audience the emotion of the scene and thus the meaning of the various interactions of the story elements. Human actors, good ones, makes this task fluid, quick, and easy.
After all, being there in a particular event would feel so much different than merely listening to someone talking about it. And even listening to a story from a more expressive story teller is so much better when compared to someone who just reports it to you without even making a gesture.
What you can preserve for certain are the character interactions and the progression of their relationships as the event unfolds. The thematic nature of those relationships will never be different, whether you make a script for a film, a comic or a game. And of course all of the fundamental elements and themes of the story itself will never be changed.
What you may be forced to add are activities that doesn't really relate to the scene at all. Say, harvest these amount of minerals. I call these player errands.
Player errands however must at least be tied into the actions or activities central to the scene we are trying to convert or transcribe into a mission scenario.
Mostly, there's a lot of running around from script to mission, and vice-versa, when it comes to making this mission, almost to the point confusion comes in that either of the two makes any sense anymore.
The quick fix to this usual obstacle, which is by the way caused by exhaustion, is to remember the fundamental elements present in your scene, what you wanted to tell the audience, and how you'd want them to react or feel.
Making a mission scenario from a scene script is just like asking the question, "How do I show the player these things?"