In “A Murder in the Park,” those views are stood on their heads. Here the crusading liberal journalists are deeply foolish or deeply corrupt, or both. The police are innocent and abused. And the bad guy really is the bad guy.

The film opens with a scene that seems too good to be true – and in fact, is. Anthony Porter, a black Chicagoan sentenced to death for killing a black couple late one night at a public swimming pool in 1982, is released from prison just when he was supposed to be executed (the wardens had already asked what he wanted for his last meal). He rushes out of the prison gates and into the jubilant embrace of his last-minute saviors: Northwestern University journalism professor David Protess and his students, who produced evidence that Porter was innocent and the crime was actually committed by another man.

As we soon learn, Porter’s 1999 release after 17 years in prison fit right into the media and political climate of the moment. Chicago newspapers had been full of stories of police corruption, falsified investigations, coerced confessions and judicial malfeasance. So the story of an innocent man almost executed was another instance of such horrors, one that in fact led to the cessation of the death penalty’s use in Illinois.

There was just one problem, though, as we hear minutes into the film: Subsequent investigation proved conclusively that Porter was guilty and that the case for his innocence was a fantasy turned into bogus evidence by Protess and his students, whose work later got an innocent man sent to prison for the murders Porter committed.

The film then jumps back to 1982 to show how the murders and their investigation unfolded originally. It was a hot August night when Jerry Hillard and Marilyn Green were sitting in the bleachers of that public pool and someone came up and shot them five times at point-blank range, then ran away. Anthony Porter was stopped by a cop leaving the scene but was not held because he didn’t have a weapon.

It wasn’t long before four witnesses – two pairs of young male friends – turned up. They had all been at or near the pool and said they had noticed Porter and another man sitting on the bleachers near the couple. They all knew Porter from the neighborhood; they describe him as vicious, violent and scary. When the four heard the shots that killed Green, they looked up and saw Porter fire his pistol at Hillard’s face and then flee.