Not every “mystery thriller” needs to have much of a mystery about it to work. But none can get by without that “Eureka,” aka “Oh (snap)” moment. And in “The Good Nurse,” that’s a doozy.

This true story, with some minor alterations, provides an acting showcase for two of the best, Oscar winners Jessica Chastain and Eddie Redmayne. They both play nurses, but only one of them is “good.” And the “aha” here is when one figures out how the other one is killing patients in their hospitals, and has done so in hospitals all over Pennsylvania and New Jersey.

Danish filmmaker Tobias Lindholm, who gave us “The Hunt” and “Another Round,” serves up a slow, deliberate story of empathetic caregivers and CYA-fixated hospital administrators, of a nurse who has become an angel of death and of dogged cops who struggle to make a case when they’re being stonewalled by the institutions where these deaths happened and continue to happen.

Chastian is Amy Loughren, a single mother of two with the overnight shift at her hospital and a secret she needs to keep for a few more months. Amy has a dangerous heart condition, and she won’t be covered by her employers’ insurance until she’s been on this job for a full year.

She is, in many ways, nursing’s ideal — talking to the coma cases, comforting family, bending the rules to make a bad situation a little easier.

Charlie Cullen also calls patients by their first name, also seems to go the extra mile when it comes to compassion. He learns Amy’s secret and he keeps it and pitches in to help her finish her trek to the “insured” finish line.

“I’m gonna help you get through this.”

But we’ve seen a “code” he was involved in at a previous hospital. And when a patient he and Amy share on their shift dies as well, we — if not she or anyone else — can do the math.

Veteran character actor Noah Emmerich and Nnamdi Asomugha (“Sylvie’s Love,””Crown Heights”) enter the picture as two cops summoned, by state health dept. mandate, to investigate what the euphemism loving “risk manager” (Kim Dickens) labels “an unexplainable incident.”

Somebody died. The hospital did a mortality report, and now, almost two months later, they’ve had to inform the police.

“The Good Nurse” has two villains — a killer and a parade of (mostly unseen) corporate suits who cover up the deaths. The film is about catching one and trying to work around or confront the other.

Administrators circle the wagons, cops lose their tempers and the title character struggles to reconcile what she’s going through, her judgement of the friend and colleague with what she’s learning.

Redmayne gives us a “quiet type” version of “the banality of evil,” not giving much away, even in the eyes. Lindholm spends little screen time showing his point of view.

Chastain has more to play and makes her character’s conflicts empathetic and understandable. This guy is saving her life and her job. He can’t be…or can he?

The police procedural element of “Good Nurse” is the most potentially riveting, and damned frustrating. But justice has proven to be a slippery thing in America in recent years, with lawyers flinging up road blocks and villains running out the clock so that even good cops can’t nab the guilty.

Lindholm’s patience with this material kind of outlasts ours. There needs to be more flesh on the bone to justify the two hour running time. The dead spots show.

And with an accompanying documentary also coming out, we can judge for ourselves if there were opportunities missed in giving the feature film treatment to this notorious case.

Rating: R (profanity)

Cast: Jessica Chastain, Eddie Redmayne, Kim Dickens, Nnamdi Asomugha and Noah Emmerich.

Credits: Directed by Tobias Lindholm, scripted by Krysty Wilson-Cairns, based on the book by Charles Graeber. A Netflix release.

Running time: 2:03