Gary Hook, portrayed by Jack O'Connell, embarks on a profound journey as a fresh recruit in the British Army, parting ways with his much younger brother Darren, who resides in an orphanage. Their bond is evident, with Gary being all Darren has left, yet he feels compelled to enlist to provide for them both.
Once at the barracks, Hook endures a grueling training regimen emphasizing the importance of unity and the motto to never abandon a fellow soldier. Soon, his squad is dispatched to Belfast in 1971, amidst the turmoil of The Troubles. Led by the inexperienced Lieutenant Armitage, played by Sam Reid, the squad is briefed on the tense dynamics between Catholic Nationalists and Protestant Loyalists in the area. The British Army has been summoned to enforce peace, but the atmosphere is fraught with hostility; Catholics view the British forces as oppressors, while Protestants stand by the crown.
As tensions unfold, Hook witnesses the harsh reality of the conflict when his unit supports the Royal Ulster Constabulary's inspections of Catholic homes, leading to intense confrontations. In a chaotic scene, a young boy seizes a fallen soldier's weapon, prompting Hook and a fellow soldier to chase after him through a hostile crowd. Despite their efforts, the mob's aggression escalates, resulting in a harrowing moment where Hook watches in horror as his comrade is gunned down by a young Nationalist named Paul Haggerty.
In a desperate attempt to survive, Hook navigates the perilous streets, narrowly avoiding gunfire while trying to blend in by swapping his military uniform for civilian attire. Meanwhile, he unwittingly stumbles upon the Loyalists in a pub where they are constructing a bomb under the guidance of the Military Reaction Force, overseen by Sean Harris and Paul Anderson. Just as he steps outside, a massive explosion rocks the building, leaving him disoriented and severely injured.
As Hook lies unconscious in the aftermath, he is discovered by two members of the local Catholic community, Eamon and his daughter Brigid, portrayed by Charlie Murphy. Despite realizing that he is a British soldier, Eamon compassionately tends to Hook's wounds, reinforcing the complex humanity that exists amid the conflict.
Meanwhile, tensions rise within the factions as Lieutenant Armitage confronts the MRF about Hook's disappearance. Eamon desperately seeks a more humane response from the IRA, reaching out to Boyle, a senior official who wishes to maintain a more moderate stance amidst the chaos. However, Hook's plight only becomes more perilous as he narrowly escapes violent confrontations, and he ultimately finds himself face-to-face with his captors led by Quinn.
When Quinn's orders to execute Hook falter due to a moment of hesitation from a young recruit, chaos ensues. Just as his fate seems sealed, Browning's forces intervene, resulting in a spiral of violence and conflict that ultimately reshapes loyalties.
Through all the harrowing twists and turns, Hook fights to survive as he is caught between warring factions, ultimately returning to his barracks with a traumatic story of survival. Despite the chaos of the situation, he is able to reunite with his beloved brother Darren, finding solace in their bond amidst the desolation that surrounds them.
