An interesting footnote to the Lincoln assassination receives sober, respectful, but rather stilted and talky treatment in “The Conspirator.” Made with scrupulous attention to external detail but lukewarm inner life, Robert Redford’s film is a television-quality history lesson that makes a point of drawing a parallel between a presumed miscarriage of justice from the nation’s past and contemporary U.S. anti-terrorist judicial practices. It may be instructive, but it’s not very compelling.

The accused co-conspirator in the president’s murder is Mary Surratt (Robin Wright), the mother of one of the accused—who’s fled and in hiding—and proprietress of a boarding house where the plotters regularly met. At the urging of Senator Reverdy Johnson (Tom Wilkinson), who’s concerned that she’s being railroaded by Secretary of War Edwin Stanton (Kevin Kline) and his military tribunal headed by General Hunter (Colm Meaney), Union Officer and lawyer Frederick Aiken (James McAvoy) reluctantly agrees to defend her, despite the long odds.

The narrative centers on Aiken’s growing conviction that Surratt might not be guilty and his efforts to prod the court into acting in a genuinely just fashion, or at least to show mercy toward his client, who faces the possibility of becoming the first woman to be executed in US history. To succeed he must overcome the influence of Seward, who’s determined to end the political turmoil caused by the assassination by swift retributive sentences against all the accused. He must also deal with the increasing animosity of the public to his efforts—including old army friends like Nicholas Baker (Justin Long), whose life he was instrumental in saving on the battlefield.

The script by James Solomon may be long on factual accuracy, but at least as directed, somberly and without energy, by Redford, it lacks dramatic urgency. Even the last act, in which there should be a good deal of tension about whether a reprieve will be granted to Surratt or not, the picture is slack. Except for Kline and Wilkinson, who give their scenes some much-needed zest (even if Kline is stagy and mannered), the cast is stuck in the doldrums too. Hidden behind a beard, McAvoy is nondescript and Wright is so passive she practically disappears. A similar lethargy afflicts Long, Meaney, Evan Rachel Wood as Surratt’s daughter, and Danny Huston as the prosecutor.

The picture is attractively mounted, with period design that belies the obviously mounted budget (the locations in Savannah, Georgia, work fine), and cinematography by Newton Thomas Sigel that gives the visuals a lush, glistening quality that bathes everything in a glow that, one supposes, is intended to be evocative. It’s pretty but has the effect of making the movie seem even more like a museum exhibit or painting than it might otherwise do.

It’s obvious that one of the purposes behind “The Conspirator” is to raise questions about how fair the current US plan to try accused terrorists before military tribunals, without benefit of the protections afforded by the ordinary legal system. True, in the Surratt case the issue was whether US citizens could be tried before such tribunals. But even though the accused terrorists don’t fall into that category, by portraying such a tribunal acting in a manifestly political fashion the film calls the integrity of the entire process into question.

That’s an important issue for people to consider. It’s a pity that “The Conspirator” doesn’t raise it in a more dramatically effective fashion. The result is a potentially sound educational tool, but not much else.