Starred Up opens in a dark anteroom where 16-year-old Eric Love (Jack O'Connell) is being processed into a prison for adults, a status he earned (the Brits call it being "starred up") due to the violence and the frequency of his crimes. O'Connell plays Eric as a near-feral survivor of abuse and neglect; his movements economical and confident, he carries himself like a cat, quick to react to a threat and prone to bursts of ferocity. Soon after arriving, Eric nearly kills a fellow prisoner who's done him no harm and then battles the guards who try to subdue him, creating a standoff by taking one man's penis in his mouth through his pants and threatening to bite it off. Though this preemptive strike is presumably intended to keep the other prisoners at bay, it has the opposite effect, earning Eric the enmity of powerful alpha dogs like one of the guards who runs the prison and the suave prisoner who unofficially runs Eric's unit and doesn't want some crazy kid causing trouble on his turf.
Eric's volatility also earns the attention of two men who want him to calm down for his own sake: his estranged father, Neville (Ben Mendelsohn), a longtime prisoner high in the pecking order, and Oliver (Rupert Friend), a volunteer who leads an encounter group whose members learn to break the cycle of violence they've been trapped in. But what first appears to be a good dad/bad dad battle for Eric's soul, with Oliver demonstrating the power of respect and affirmation while Neville fails miserably with his hectoring and beatings, turns into something more interesting as both men exhibit more—and more complicated—aspects of their characters.