They are accused of 'plotting' the assassination of the prime minister. = They are accused of 'plotting' to assassinate the prime minister.
As the 'plot' unfolds [=as the story continues; as things happen in the novel, movie, etc.], we learn more about the hero's family.
He is a master of 'plot' [=he is very good at thinking of interesting stories], but his characters are not realistic.
(..) have been added to them. The Intertitles appear in their original font. Intertitles with a different typeface (like this one) have been added to summarize the plot of missing segments. Black film stock indicates shorter sections of missing footage. An Ufa production Distributed by Parufamet. (..)1927 Metropolis