Here’s what “The Little Things” is, and what it isn’t. It’s a serial killer drama about a couple of cops on the trail of who they think is committing the crimes. It’s not an exploitative story that shows the horrific murders happening. It spells out how the two cops, each of them with very different approaches to their work, get pretty close to being in sync on the case. It’s not a cop-buddy movie. It presents a lot of questions and delivers a lot of answers. It doesn’t wrap things up in a nice, neat package at the end.

Written and directed by John Lee Hancock (“The Founder,” “Saving Mr. Banks,” “The Blind Side”) and set in California in 1990, the film’s plot starts small, then becomes complex as characters, exhibiting their strengths and weaknesses, keep getting more and more challenges to deal with.

Joe Deacon (Denzel Washington) is a small-town deputy outside of Bakersfield, who is sent on a lowly evidence-gathering assignment to Los Angeles, where he used to be a big city detective. Five years earlier, he was stationed in the LA HQ where he’s now mumbling hello to former co-workers. But his reputation was marred back then, and the only friend he still has there is a forensics scientist named Flo (Michael Hyatt), and even she seems to be slightly put off by his presence.

Jim Baxter (Rami Malek) is the slick, no-nonsense up-and-coming LA detective who is mired in an out-of-control series of murders (two months, four victims, no suspect). He has heard of Joe, thinks little of him and, when they meet, can only come up with, “Things have changed a lot here since you left.”

The first half of the film is about these two guys, how they kind of dance around each other because they’re such different people, and about the events that lead to them attempting to put aside those differences in order to catch a killer. Yet it’s not all that straightforward. Five years ago, Joe was working on a serial killer case in LA, a case that so obsessed him, he was suspended, then divorced, then suffered a heart attack, then was demoted and transferred. The case was never solved.

Now Jim is having trouble stopping a serial killer in LA, and Joe, just visiting to pick up unrelated evidence, believes there are similarities in the two cases. Could he figure out a way to stay in town, join up with Jim, help solve his case, and maybe also solve the same one that’s been haunting him since he left?

He does stay. And the snappy, right to the point dialogue, along with some fast-paced but never too-fast editing gets everything cooking early on. Another murder is committed, then a few more; Joe starts going through brief, disturbing flashbacks of working on the 5-year-old crimes; both cops have seen too many dead bodies in their time on the force; and they team up to catch the perpetrator, even if they don’t intend to play quite by the rules.

The opening moments of the film’s second half introduce a creepy fellow named Albert Sparma (Jared Leto), who is immediately pegged as a suspect. Brought in for questioning, he uses his lifeless, but somehow piercing eyes, his relaxed body language, and his sing-songy voice to play with the heads of everyone in authority around him.

The ideas that begin to inexorably draw together are that no one really knows if Sparma committed any crimes, that Joe is weary and haunted, and that Jim is slowly becoming as obsessed with this case as Joe was with his back when he was a detective.

There’s a lot of sitting around and waiting, and a couple of instances where the film breaks into action, out on the road. But it’s mostly an exploration of three characters via some terrific interior performances. We see and hear what the two cops and the possible killer are doing and saying, but we’re constantly wondering about what they’re thinking. Make that four characters, as Flo, who returns late in the film, is also keeping something locked up inside.

An odd couple of endings clear some things up, while keeping others a bit foggy. A neat coda has one character commit a selfless act. All of that is part of what makes the film stand out.

“The Little Things” opens in selected theaters and premieres on HBO Max on Jan. 29.

Ed Symkus can be reached at esymkus@rcn.com.

“The Little Things”
Written and directed by John Lee Hancock
With Denzel Washington, Rami Malek, Jared Leto, Michael Hyatt
Rated R