Mary Surratt, the owner of the boarding house where John Wilkes Booth and his co-conspirators stayed, was swept up in the dragnet following President Lincoln’s assassination. Frederick Aiken (John McAvoy), a Union veteran fresh from law school, is asked to defend Mary. He begrudgingly takes the job, and in the glare of a military tribunal he is transformed from glib attorney to passionate defender of her rights and champion of Constitutional values. But The Conspirator might as well be called The Defender, since Robin Wright’s Mary has little more to do than stoically await her fate. The story of her imprisonment, trial, and hanging (she was the first woman to be executed by the U.S. government) makes a solid if sometimes airtight courtroom drama, and Robert Redford’s often bloodless, slow-grind direction actually works in his favor here. Still, the movie feels trapped in the rarefied air of period correctness, while deftly portraying a complicated attorney-client relationship and shining a lens on the assassination’s aftermath.