Nineteen-year-old inmate Eric (Jack O’Connell) gets transferred to a prison for adults, where he must learn about new power dynamics and survival skills, in “Starred Up.” (Tribeca Film)

Starred Up,” an engaging prison drama featuring a standout ensemble of British and Australian actors, gets its title from a term describing youthful offenders who are sent to adult facilities for violent behavior. As the film opens, its 19-year-old protagonist, Eric (Jack O’Connell), is in the midst of just such a transfer, to a prison where immediately upon claiming his cell he fashions a homemade shiv from a disposable razor and a toothbrush.

Such are the vivid, atmospheric details that give “Starred Up” its vigor and credibility, even when its plot goes a bit wobbly with schematics. O’Connell delivers a breakthrough performance as a young man who, it becomes clear, has suffered from lack of parenting, predatory “care” and florid systemic failure. After establishing his bona fides by brutally beating a fellow prisoner, he becomes subject to an emotional tug of war between a prison elder (Ben Mendelsohn) and a well-meaning group therapy leader (Rupert Friend) who encourages his charges with such jargony catch phrases as being “on a journey together” and “Stay with it, breathe with it.”

Those moments teeter perilously close to “I can reach these kids!” self-righteousness. But “Starred Up” earns its own bona fides, both as a character study — O’Connell, Mendelsohn and Friend are all terrific in their roles, as is their supporting cast — and as an oblique but candid portrait of how prison works. The film’s writer, Jonathan Asser, worked in the prison system as a therapist, which might account for the script’s too-obvious gear-work, but surely also explains its insights, from the power dynamics between alpha prisoners and their putative guards to the portrayal of incarceration as something akin to extreme day care, complete with juice boxes and jammy-like fleece sweat pants.

Seen through Asser’s lens, Eric and his fellow convicts aren’t beasts as much as big babies — albeit ones more prone to process scary emotions with acts of violence than by using their words. For all its savagery and hopelessness, “Starred Up” manages to be sympathetic, not only because of O’Connell’s galvanizing turn, but also Asser and director David Mackenzie’s unwavering commitment to portraying his character with as much compassion as brutal honesty.

★ ★ ½

Unrated. At AFI Silver. Contains profanity, graphic violence, nudity, smoking and adult themes.
106 minutes.