Following the assassination of Abraham Lincoln, the country was eager for someone to pay for the deed. John Wilkes Booth, the man who actually shot the President, was tracked down hiding in a barn and killed by a Union soldier, who acted on his own.
Associates of Booth were rounded up, arrested and put on trial in a Military Tribunal. Besides the men who conspired with Booth, they also arrested Mary Surratt (Robin Wright), the owner of a local boarding house. Her son was among the followers of Booth and as they gathered in her house to plot the kill, the government was sure she was part of it.
Mary's son takes off in hiding, and she's put in a cell to await her fate. The Secretary of War Edwin Stanton (Kevin Kline) gives the job of defending her to Reverdy Johnson (Tom Wilkinson), but because of his Southern roots, he believes that Frederick Aiken (James McAvoy), an earnest young Union Captain with little experience, should take her case.
Aiken believes her guilty, and as she does little to help in her own defense, he has to try to plead with her for cooperation. Was she aware of the dark plans developed in her own rooms, or was she quiet to protect her son - those are the questions that her defense attorney needs to find out.
This is a serious account of the historical event and the turbulent aftermath. It doesn't heavily push an agenda of her guilt or innocence, the film lets you come to that conclusion by yourself.
Wright and McAvoy give strong and moving performances. Historical films have to do a couple of things to make them special. For me, they have to tell me something I didn't already know, and make me want to find out more - this film does both.