Robin Wright portrays Mary Surratt ably, as a Maryland widow who moves to Washington to support herself and her children by taking in boarders (as was common at the time) and gets swept up in the doings of John Wilkes Booth through her son John, a Confederate courier. James McAvoy portrays one of her appointed attourneys - and a Union veteran - Frederick Aiken (another of her lawyers, John Clampitt, is not included in the movie, probably to keep from bogging down the plot in too much detail). Aiken is initially reluctant to defend her, but he comes to believe in his client's innocence. Initially, Booth, John Surratt, and other conspirators seek to kidnap and ransom Lincoln. But when Lincoln proposed giving some freedmen the right to vote, Booth's plan changed to murder. How much Mary knew of either plot, and whether or not she was much involved, is still debated today. The movie is largely sympathetic to her, but it still allows the audience to judge for themselves her guilt or innocence. To experts on the subject, there are some glaring inconsistencies with the original events: the omission of Mary Surratt's other priest, the appearance of the Old Penitentiary Building (it didnt have a moat), and the appearance of some of the key players themselves.
"The Conspirator" draws some contemporary parallels about how people in power can use fear after cataclysmic events (in this case, the end of the Civil War and Lincoln's assassination) to justify drastic measures against civilians, whether or not they be guilty.
You can watch the trailer for "The Conspirator" here: