Edi Gathegi plays a man staring at a life sentence through the dingy glass of a solitary confinement cell in “Caged,” a thriller about what this sort of isolation does to someone’s mind.

It’s a fairly run-of-the-mill tale from “inside,” distinguished by its myopic, paranoid setting and inmate’s-eye-view of his shrunken world, and by a sharp cracking-up turn by Gathegi (“Princess of the Row” and “X-Men: First Class”).

The casting of this minimalist mystery isn’t uniformly up to Gathegi’s standard, and the plot is thin on “surprises.” But co-writer/director Aaron Fjellman at least runs us through the tropes of the genre at a fairly decent clip.

Harlow Reid is on the phone, trying to be patient as he reaches out to his lawyer about something that’s happened in jail. Actually, he’s trying to be patient with a receptionist who keeps saying “I’m sorry.”

Impressing on her the urgency that he’s facing “a life sentence, and I’m innocent,” and that something happened which has now landed him in solitary proves impossible. The firm has dropped the appeal, which has been “postponed indefinitely.”

If he thought he was on his own before…

Reid (Gathegi) is a doctor it turns out. He was accused and convicted of murdering his wife (Angela Sarafyan, all no-energy and no spark). Flashbacks show us varying versions of what happened that day on a sailboat they’d just bought, and inter-titles show us how many days Reid has been inside.

The “war of wills” here involves a cruel, scarred prison guard, played by Melora Hardin (TV’s “The Office”) at her most sadistic. What’s this piece of a razor blade doing inside your mattress, convict?

“If you were as smart as you think, you wouldn’t be here!”

The guy in the solitary cell next door keeps flipping out and flooding “The
Shoe” (that section of the block). Harassed by the guards, his rations somehow reduced, beaten in the showers by other inmates and badgered to “confess” by the Catholic warden (Tony Amendola, very good) who promises a “special program” he can get the guy into once he’s “admitted” his “guilt,” we wonder how long it’ll be before Reid is hallucinating, screaming and stuffing socks into the toilet to flood the place and get the guard’s attention.

We can kind of piece together what “really” might be going on here through snippets of dialogue, the father-in-law wealthy enough to bankroll Reid’s practice, perhaps wealthy enough to ensure judicial “revenge,” arguments with the wife that may have turned violent, the guard’s reason for her sadism.

Gathegi gives us a fraught representation of desperation that twists into madness — rattled at the mayhem he hears but doesn’t see the psychotic next door endure, seeing “faces” on the concrete walls, freaking out further as he has to improvise a way to write his letter of appeal to the judge.

We’ve seen much of this before, and as interesting as Gathegi makes this guy — stream “Princess of the Row” when you get a chance — the lack of surprises in the various waypoints of the story rob the twists at the end of any impact.

MPA Rating: unrated, graphic violence

Cast: Edi Gathegi, Melora Hardin, Tony Amendola, Angela Sarafyan

Credits: Directed by Aaron Fjellman, script by Aaron Fjellman, James ‘Doc’ Mason. A Shout! Factory release.

Running time: 1:21