In County Cork, Ireland, during the tumultuous year of 1920, Damien O'Donovan is on the verge of leaving his rural hometown to pursue a career in medicine at a hospital in London. However, his path takes a dramatic turn when he witnesses the brutal execution of his friend Micheál Ó Súilleabháin at the hands of the British Black and Tans for defying orders. This shocking event deeply unsettles him, prompting him to reconsider his decision to forsake his homeland. Although he initially resists the urging of friends to join the Irish Republican Army (IRA), saying the fight is futile, Damien ultimately chooses to stay and is inducted into the IRA's local brigade led by his brother, Teddy (Liam Cunningham).
After training in the rugged mountains, the brigade embarks on a mission to storm the local Royal Irish Constabulary barracks for firearms, subsequently using these weapons to execute four Auxiliary members. Tensions escalate when Sir John Hamilton, a powerful Anglo-Irish landowner, employs one of his servants, Chris Reilly, to relay intelligence to the British authorities, leading to the arrest of the entire brigade. While incarcerated, Damien encounters Dan, a train engineer and union advocate, who shares his socio-political beliefs.
The saga intensifies as British officers subject Teddy to horrific torture, pulling out his fingernails when he refuses to divulge information about the IRA's operations. In a twist of fate, a British soldier of Irish descent, Johnny Gogan, aids in their escape, although not all prisoners are freed. As the betrayal by Sir John and Chris is exposed, both men find themselves captured by the IRA. With Teddy recovering from his injuries, Damien is placed in a position of command. The situation worsens when news arrives that the remaining IRA prisoners have endured brutal torture followed by execution, compelling the brigade to act against their foes decisively.
Caught in a moral quandary, Damien ultimately finds himself in a position where he must execute Chris—his childhood friend—and Sir John, with the orders to eradicate the spies. Soon after, the brigade successfully ambushes an Auxiliary convoy, prompting a retaliatory assault by the British on the farmhouse of Damien's love interest, Sinéad (Padraic Delaney), a member of Cumann na mBan. Sinéad endures a horrifying ordeal as her house is burned, resulting in a traumatic attack that sees her suffering physical harm.
The cultural landscape transforms after the signing of the Anglo-Irish Treaty, which offers Ireland Dominion status within the British Empire, yet sparks a division among the brigade. As the conflict intensifies between those advocating for the Treaty and those yearning for a fully independent Republic, the tension reaches a boiling point. Teddy argues for acceptance of the Treaty to secure peace, while Damien and Dan demand a more radical approach, advocating for the collective control of industry and agriculture.
With the establishment of the Irish Free State, Damien and his faction resist the new order and align with the Anti-Treaty IRA. The eruption of the Irish Civil War leads them to initiate guerrilla warfare against the Free State army, inciting fear in Teddy of possible British invasion if the republicans gain strength. As conflict escalates, tragedy strikes when Dan is killed, and Damien is apprehended during a raid, culminating in his death sentence.
As he awaits execution beside Teddy, who commands the firing squad, the brothers face an agonizing reckoning. Teddy, desperate to save Damien, implores him to betray his allies in exchange for amnesty and a life with Sinéad. Bruised by this proposition, Damien proudly refuses to "sell out" the Republic. In his final moments, he pens a heartfelt letter to Sinéad, reiterating his unwavering beliefs and quoting Dan's poignant reflection: > "It's easy to know what you're against, quite another to know what you're for." Damien meets his fate at dawn, showing resolve and unyielding love for his country. In his grief, Teddy delivers Damien's farewell to Sinéad, who, consumed by sorrow, lashes out in anger and heartbreak as she confronts the tragic reality of their divided lives.
