For four seasons, Viola Davis has starred for ABC in "How to Get Away With Murder." Her new series for the network, "The Last Defense," might well be called "How to Get Convicted of Murder."

The seven-part documentary, which airs Tuesday on ABC at 10 p.m., reargues cases of two prisoners awaiting execution. It begins with Davis (one of several executive producers) saying in a voice-over that every year an average of five prisoners on death row are exonerated.

"The Last Defense" may or may not add to that number. But it offers a powerful argument that people can be convicted as much by emotion and prejudice as by evidence.

The first four episodes re-examine the 1997 murder conviction of Darlie Routier, a Texas woman whose two young sons were stabbed to death.

Routier called in the killing to 911, saying that an intruder had attacked her and the boys while they slept. (We hear the excruciating audio several times.) Though Routier was herself wounded badly, including a life-threatening slash to the throat, prosecutors soon charged her.

Experts for the prosecution argued that Routier's knife wounds were self-inflicted and that blood spatter and broken glass found at the crime scene was inconsistent with her account.

The documentary also notes a significant exculpatory piece of evidence - a sock, stained with both boys' blood, found down an alley from the house. "The Last Defense" suggests that investigators minimized it because they were already invested in Routier as a suspect.

But the most powerful part of the case against Routier - and the most disturbing, in the retelling - was an all-out character attack. Prosecutors painted her as a materialistic, "self-centred" woman for whom children were "an impediment to the good life."

"The Last Defense" is plainly a work of advocacy. But it's sensitive to the drive for justice. It's hard not to see the home-video footage of Routier's sons, the blood stains on the Power Rangers bedsheets, and not want someone to be punished. "The Last Defense" questions whether that desire can overwhelm the evidence.

The last three episodes cover the case of Julius Jones, an Oklahoma college student sentenced to death at 21 in a carjacking murder. The series suggests a theory that Jones may have been set up for the crime by an acquaintance and examines the potential role of racism in his trial: an African-American man accused of killing a white man in the suburbs.

The episodes also tell a more depressingly familiar story: that Jones, regardless of his innocence or guilt, did not receive close to the best possible defense from the public defenders who represented him and that the system rigidly resists admitting any possible mistakes.

"The Last Defense" comes amid a recent boom in true crime, including Netflix's "Making a Murderer" and its expansion of the 2004 documentary "The Staircase," as well as HBO's "The Jinx" and the podcast "Serial."

"Defense" is more straightforward than some of these contemporaries, which are often produced as literary tales about character and the fluidity of narrative. It doesn't have the wealth of original video available to Netflix's "Wild Wild Country" (instead, it often relies on re-creations); it doesn't offer the breathtaking twists that the makers of "The Jinx" or "The Staircase" lucked into.

What "The Last Defense" does have is a clarity of moral purpose. Whether or not you find its arguments in these specific cases exonerating, it makes a disturbing argument about the potential for error in capital cases.

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For decades, TV legal procedurals have offered audiences the promise of certainty. They frame legal cases as puzzles that the police and prosecutors shape into a definitive picture. This portrayal has served law enforcement - "Dragnet," for instance, was partly the result of police public relations efforts - and it arguably has shaped the attitudes of audiences, both in the living room and the jury box.

Like some of its true-crime predecessors, "The Last Defense" is a kind of anti-procedural. It asks the audience instead to confront uncertainty and to ask how willing we are to accept that in real nonscripted life the system can get the answer wrong.