The correct preposition to fill the gap is with, making the phrase ‘concerned with the plot’.

১. ‘Concerned with’ means being involved in or associated with a particular activity or situation.
২. ‘Concerned for’ is used to show worry or anxiety about someone’s well-being.
৩. In the context of a ‘plot’ or conspiracy, ‘with’ accurately reflects the involvement of the men.

অতিরিক্ত তথ্য: Prepositions change the entire meaning of the verb depending on the object.