“Root Letter” is an indie murder mystery about a missing pen pal, a film based on a Japanese video game.

It’s a slow, sloppily-structured exercise in tedium that is simple yet hard to follow, short but interminable thanks to leaden pacing and often-mumbled dialogue by its cast of mostly-unknowns.

Nah. Not beating around the bush on this one.

An opening scene introduces us to a teen we see getting a beat down thanks to an enraged father catching young Carlos (Danny Ramirez) having sex with his daughter.

That puts Carlos in a Tulsa hospital. And that’s where pen pal notes from Sarah (Keana Marie) are forwarded. It seems his class in Tulsa and her class in Baton Rouge have been assigned randomly-selected pen pals to correspond with as writing exercises.

As they’re both well into high school, this seems oddly late for them to be getting such elementary writing instruction. Usually this sort of assignment rolls out for pre-hormonal fifth graders.

They share the banalities of life — sanitized daily routine, childhood memories. She’s into Billie Eilish and Lorde, he’s all about Slayer and Metallica.

He’s just now figuring out “you can’t save everyone,” he writes. “Dear Carlos, I killed him,” she responds.

When Carlos recovers from his injuries and settles back into work washing dishes, the letters stop. That’s when he decides to travel to Louisiana and figure out what happened.

As as bland as this picture has been — sharing little snippets of his life, cutting to show bigger slices of hers — it’s about to get a lot worse.

Director Sonja O’Hara and screenwriter David Ebeltoft never get a handle on how to handle parallel structure in a screenplay. We lose track of Danny in the fictive “present” for most of the movie, and what few scenes there are utterly blow the mystery of how a teen would figure out another teen’s last name and track her down. Google isn’t the answer.

Danny spreads her letters on his bed as if to hunt for clues, but nah. Let’s just go find her English teacher and get her to figure out a way to let him know without violating her school system’s ethics and without making punching-bag Danny — he gets pummeled a few times — break a sweat doing his own homework.

Sarah’s mother (Lydia Hearst) seems a wreck, and Sarah’s whole life with her is an “Is he gone?” work-around with Mom’s rotating collection of boyfriends and what turn out to be her addictions and probation problems.

Sarah’s friends aren’t much of an escape either. BFF Zoe (Kate Edmonds) has hooked up with Mr. Wrong. And Jackson (Sam A Coleman) is interested in getting into the drug trade, using their stammering, supposedly meek mutual friend Caleb (Breon Pugh) to steal drugs from Caleb’s over-armed, camo-loving drug-dealing uncle (Mark St. Cyr).

Another friend slept with Sarah’s now-ex boyfriend. So Sarah gets drunk at a party, and the film is so sloppy we can’t tell if it’s from imbibing or it she was roofied. Next thing she knows, she’s awakened in a nice house down the street where the kind couple (Terry J. Nelson, Dodie Brown) took her in and let her sleep it off.

Turns out, they lost a daughter about her age. It made the wife a little crazy and left them both shattered. Could they be Sarah’s lifeline?

All this back story isn’t presented as something Carlos is figuring out, reading up on or being told by the good folks of Baton Rouge. Because remember, the script has forgotten about him. Until, that is, he starts poking around wherever Sarah might have been and asking questions about what became of her.

Hand to heart here, very little of this makes any sense. Whatever happened to Sarah would have generated a police report and journalism Carlos could access. Whatever he’s getting off TV station websites doesn’t explain in the least where she is — above or below ground.

Sarah’s an interesting character, and those she finds herself throwing in with are at least colorfully bad. The high school milieu has some limited interest. But Carlos is dully written and sleepily played.

I watch movies for a living, and as thrillers go, I found this laughably inept at just getting the basics of storytelling down. You need to understand who and how people are, follow a narrative throughline of some sort and give us something to hang onto so that we’re in the same boat as the protagonist trying to piece this “mystery” together.

The parallel structure problems, a third act string of crimes without consequences or even remorse and the idiotic steps Carlos takes in tracking somebody he’s never met with only a few PG letters to go on overwhelm this movie and don’t exactly embellish the image of the game it’s based on, either.

Honestly, I feel I know less about what connects these two and why any of this is worth exploring than I did when “Root Letter” started. I know I care a lot less.

Rating: unrated, violence, drug abuse, sex, profanity

Cast: Keana Marie, Danny Ramirez, Lydia Hearst, Mark St. Cyr, Sam A. Coleman, Breon Pugh, Kate Edmonds, Terry J. Nelson, Dodie Brown

Credits: Directed by Sonja O’Hara, scripted by David Ebeltoft, based on the Kadakowa Games video game. An Entertainment Squad release.

Running time: 1:26