The conspirator of the film’s title is Mary Surratt (Robin Wright), the woman who owned the boarding house where the accomplices met in secret and whose son was closely tied to Booth. Blinded by revenge and an unrelenting desire to put the matter to rest, the American State Department completely disregarded the rule of law and Surratt’s constitutional rights in their fervor to secure a conviction. James McAvoy deftly plays Frederick Aiken, the attorney assigned to defend Surratt and who ends up fighting overwhelming opposition from the seats of power in his quest for a fair trial.