In 1922, Joe O'Reilly (Ian Hart) seeks to comfort Kitty Kiernan (Julia Roberts), who is overwhelmed with grief following the death of Michael Collins (Liam Neeson). The narrative then shifts back to the tumultuous finale of the Easter Rising in 1916, where Collins, alongside Harry Boland (Aidan Quinn), Éamon De Valera (portrayed by Alan Rickman), and other patriots surrender to British forces. Determined not to follow the oppressors' lead again, Collins tells Boland, >“We won’t play by their rules, Harry. We’ll invent our own,” foreshadowing the battles yet to come.
As the Dublin Metropolitan Police intensively search for revolutionary leaders, the tragic fate of Collins’ companions unfolds, leading to all but De Valera, an American citizen, facing execution by firing squad. Following his confinement, Collins finds himself in Frongoch internment camp with Boland and others, where they begin plotting their next moves. Upon his release, Collins runs for election in the First Dáil but faces fierce violence from the Royal Irish Constabulary during a rally, where Boland comes to his rescue.
While recuperating at a friend's farm, Boland and Collins encounter Kitty, which sparks a romantic involvement between Kitty and Boland. Meanwhile, Collins learns from Detective Ned Broy (Rea) about the impending arrest of De Valera and his cabinet, but the latter insists on staying public, believing it’ll force their release. Only Collins and Boland escape the dragnet, and as a result, no outcry arises.
Collins and Boland ultimately embark on a mission to England, successfully freeing De Valera from Lincoln prison. However, tensions rise as De Valera, feeling overshadowed by Collins, plans a trip to the United States to seek support from Woodrow Wilson, bringing Boland along for the journey. Left to command the operations in their absence, Collins directs the IRA to intensify attacks on police barracks to seize weapons and issues a stern warning of death to those who collaborate with the British.
On Bloody Sunday, a tragic turn occurs as Collins' squad assassinates fourteen individuals from MI5's Cairo Gang, provoking retaliation from the Black and Tans, who indiscriminately fire on a crowd attending a Gaelic football match at Croke Park. In the aftermath, Broy meets a grim fate for destroying documents.
When De Valera orders an attack on The Custom House, Collins raises concerns that a conventional conflict would disadvantage them, but the Irish Cabinet stands by De Valera's directive, leading to disastrous consequences: six men are killed, and seventy apprehended. Feeling the pressure, Collins privately confides to Boland that their position is dire, yet the unexpected occurs as the British soon initiate a cease-fire.
In London, Collins reluctantly engages in negotiations despite his reservations about being a diplomat. The signing of the Anglo-Irish Treaty sparks controversy when De Valera reacts vehemently to the disclosure of terms without his input. Collins argues passionately that the Treaty represents a stepping stone to their republic.
As political tensions escalate, both Collins and De Valera campaign for support among the Irish populace. Collins narrowly escapes an assassination attempt but proposes to Kitty, who joyfully accepts his proposal. However, when a vote favors the Treaty, De Valera's refusal to concede leads to volatile clashes, with the IRA taking control of the Four Courts in Dublin.
Appalled by the prospect of engaging former comrades in battle, Collins, along with the Cabinet, realizes that failure to act will invite British intervention. The Battle of Dublin ensues, resulting in the IRA’s retreat from the city. In a heartbreaking twist, Boland is fatally shot while attempting to evade capture.
Crushed by Boland’s fate, Collins makes a heartfelt plea for peace through intermediaries, unwittingly setting a meeting with De Valera at Béal Na Bláth. Tragically, a surprise ambush occurs as government troops are attacked en route, and Collins is mortally wounded in the chaos. His life slips away in Joe O'Reilly’s arms, and as Kitty prepares for their future together, she receives the devastating news of his death just moments after trying on her wedding dress. In a bid to console her, O'Reilly reflects on Collins’ wishes, urging Kitty to move forward from her grief.
