“In the Heat of the Night” is one of those classics that does not fade in the memory. Yes, it’s a thriller with a murder mystery at the heart of it. But “whodunnit” is immaterial to the film’s thrills, and the one thing I seem to forget every time I watch it anew.

Re-watching this five-time Oscar-winning time-capsule of the 1960s South during Oscar season is a reminder of how sometimes the Academy gets it right, even on a Best Picture winner which they made a collective blunder on that hangs over it more than half a century later.

How in the Hell did they not nominate Sidney Poitier for this, the jewel in his acting crown? Even if he had a statuette already on his mantelpiece, the omission is as glaring as the film’s Southern Racism — then and now — messaging.

Senior citizens, especially in the South, still gravitate to the Carrol O’Connor/Howard Rollins TV series that took this movie and watered it down for 1980s and ’90s audiences, almost a “post racial” take on a movie that was all about race. But Norman Jewison’s film exists on a whole other plane, an acting showcase that made the smartest cop, the most educated and articulate man in town a Black visitor, played by Poitier at his Matinee Idol peak. So yeah, he was the best looking man in town, too.

An outsider with money and a plan for a factory that will bring jobs to backward Sparta, Mississippi is murdered. The gum-snapping police chief (Best Actor Rod Steiger) is new enough on the job to be frantic for suspects.

One of the people rounded up is a Black man in a suit. It’s hard to imagine the jolt this movie provided to audiences in the ’60s when it turns out the visitor, waiting for a train to go back home, is a Philadelphia police detective. He withholds this information from the yahoo cop (Warren Oates) who picked him up, and delays it just long enough to humiliate the racist chief.

The film’s genius is having the chief self-aware enough to recognize, based on a couple of the Black man’s simple observations, that this is a much smarter cop than him. And if he won’t admit it freely, he’s sure as shooting talking Det. Virgil Tibbs’ Philadelphia boss into making him “assist” in the investigation.

Tibbs, through gritted teeth, shows up the cops, the “doc” doubling as coroner, and the South in general with his professionalism and willingness to put personal antipathy aside in investigating this murder.

But for the first time ever, the “noble” Black man archetype has an edge. There’s only so much he’s going to take. An infamous slap famously is returned in kind, his unspoken disdain for “sleepy time Down South” laziness and small town venality and incompetence, Poitier’s Tibbs was an indictment of “the way things have always been.”

It’s left to the wealthy industrialist’s widow from “up north” (Lee Grant) to say out loud what Tibbs must be muttering under his breath.

“My God, WHAT kind of people are you? What kind of place is this?”

There will be no railroading the first “likely” suspect (Scott Wilson of “In Cold Blood,” “The Right Stuff,” “Junebug” and “Monster”). Not this time. Not with a sharp cop who can see through the prejudices and police “profiling” and general eagerness to grab someone and make the charge stick, no matter what.

Watching the film now, with the racial slurs often edited-out even though white supremacists and Twitter have brought them back, it’s impossible to miss the messaging that the film still hammers home. Nothing will change for the better in the still-backward corners of the country until bigotry is set aside and every voice heard from, every citizen’s potential is allowed to flourish.

Steiger makes Chief Gillespie a simmering stew of resentment, sarcasm, outrage and panic. He’d like nothing better than to drop a few more “n words” on Tibbs and pack him on his way, maybe even let the redneck mob have a go at him.

But no, he needs him, and Steiger never lets the viewer forget how much that infuriates and dismays him.

Poitier didn’t get an Oscar for this, one of the all time great screen performances. But that seems small potatoes 55 years later. Wherever else his career had been, wherever else it would go, “In the Heat of the Night” would make him an icon, mourned the world over when he passed away last year.

He was a big enough star to ensure this movie was not filmed in the still-deadly-for-Black-people South, so Sparta, Illinois doubled for Sparta, Mississippi. He fought to ensure the slap heard all over America stayed in “every print” of the movie, no matter where it played.

And he is the reason to try and talk the elderly relatives into eschewing the namby pamby murder-of-the-week TV version of “We all get along now” Sparta. If Poitier’s name isn’t over the title, and Ray Charles isn’t singing the title tune, move along.

Rating: “approved,” violence, innuendo, racial slurs

Cast: Sidney Poitier, Rod Steiger, Lee Grant, Warren Oates, Larry Gates, Scott Wilson, Matt Clark, Quentin Dean, Arthur Mallet, Larry D. Mann, William Schallert and Anthony James.

Credits: Directed by Norman Jewison, scripted by Stirling Silliphant, based on a novel by John Ball. A United Artists release on PosiTV, FreeVee, Tubi, Amazon, etc.

Running time: 1:50