Sometimes, when you get an edit of a movie to work on it might be highly incomplete, with lots of placeholders, storyboard cut-ins etc. so it might be tricky to actually understand the story or important details. Also, sometimes, the storyline will just be too complicated or maybe even badly written so that it is extremely tricky to follow what is going on. However, of course as the composer you should understand it, you should know the motivation behind all characters etc. so you can translate it properly to music. If you don’t understand something, don’t be too shy to ask! It is not a display of your stupidity or being scared of critzising your employer’s work but actually many filmmakers are very thankful for fresh input regarding the understandability of their storytelling so they might be able to fix certain things before the release. If you are not sure about a certain direction the story takes, just let them explain it to you. Especially on tricky plots with many twists, you might be able to help clarifying things for the audience with the music, so it is particularly important there that you have a clear overview over the story.