Canadian director Atom Egoyan dramatizes the West Memphis murder case of the early 1990s, in which three misfit teens were convicted — and one sentenced to death — in the murder of three young boys. Several documentaries helped establish the teens’ near-certain innocence; they were freed after 18 years in prison.

“Devil’s Knot” ends after the trial, so this movie is the tale of a Salem-style lynch mob. It’s as preachy as “The Crucible,” with the score permanently set on “mournful.”

Colin Firth plays a real-life investigator whom the script renders as noble as Atticus Finch. Reese Witherspoon does haunting work as a victim’s mom. But the stately pace and the faultless art direction add to the impression that truth was not only stranger, but more dramatic.