When government officials couldn't prosecute Lincoln assassin John Wilkes Booth (he was shot to death), they staged a kangaroo court to convict those guilty of association with him.

One was Mary Surratt, in whose Washington boardinghouse Booth hatched the plot to kill the president. In a rush to injustice, she was the scapegoat sacrificed to feed those hungry to avenge Lincoln's death. Her son, John, was a known associate of Booth. And it would appear that she was tried because her son was nowhere to be found.

Surratt's story, brought to life in a stoic and sympathetic performance by Robin Wright, is a little-known chapter of American history recovered by screenwriter James D. Solomon and director Robert Redford in The Conspirator.

This provocative account of a war-weary administration that denied Surratt her right to a fair trial starts slow but builds momentum in the scenes with Wright and Evan Rachel Wood as Surratt's flinty daughter, Anna.

"In times of war, the law falls silent," goes Cicero's maxim, quoted in the film by Surratt's prosecutor, Joseph Holt (Danny Huston). Her reluctant lawyer is Frederick Aiken (James McAvoy). He's a Union war hero assigned to defend the Confederate sympathizer who refers to the late commander in chief as "your president." Though she is a civilian, she is tried by military tribunal.

Initially, Aiken is embarrassed by his assignment. Friends think he's defending the enemy and shun him. But Aiken's belief in Surratt's constitutional right to a fair trial - and, increasingly, in her innocence - makes this a potent drama about a time not unlike the politically polarized present when prisoners at Guantanamo await trial for 9/11 conspiracy.

Shot mostly in enclosed spaces, jail cells, and courtrooms, Redford's film lacks oxygen but compensates for this deficit in Newton Thomas Sigel's atmospheric cinematography, which has the celestial light, sepia tones, and compositions of 19th-century photographs.

While this has the effect of making the film seem more pictorial than cinematic, it also focuses the mind on the miscarriage of justice. Redford always gets the best out of actors, and his cast - especially Wright, Wood, Huston, and Tom Wilkinson - is exceptional.

Redford's directorial hand is most evident in a sequence in which Anna Surratt is brought to the witness stand and the court orders soldiers to crowd around Mary Surratt so she cannot clap eyes on her daughter. I swear I could hear Robin Wright's heart crack.

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, "Flickgrrl, at http://www.philly.com/flickgrrl/.