An Irish rebel leader becomes addicted to violence.

In 1921, young American medical student Kerry O'Shea, whose Irish father was killed in an uprising against the British, comes to Dublin, and soon helps weapons smuggler Eileen O'Leary hide from the "Black and Tans," an armed force sent by Britain to surpress the Irish rebellion, Kerry tells her that although his heart is with the cause, he does not believe that "violence ever solved anything," and later he remarks to his nationalistic roommate, Paddy Nolan, that the rebellion is "your war." As the two friends leave a pub one evening, Kerry tries to help a terrorist who has been shot by a Black and Tan soldier. While dragging Kerry out of danger, Paddy is also shot, whereupon Kerry takes him to the home of a rebel sympathizer. There he learns that one of his professors, respected surgeon Sean Lenihan, is "the Commandant," the second-in-command of the rebel army. Despite Lenihan's best efforts, Paddy dies, and Kerry, having dropped a textbook inscribed with his name at the scene of the shootout, finds himself a wanted man. Lenihan takes him to "headquarters," where he meets "the General," who knew his father. Kerry is then taken to a small farm overlooking the ocean, to await the boat that will take him out of the country. There Kerry is introduced to Chris Noonan, a gentle poet who writes in Gaelic; Kitty Brady, a feisty barmaid who offers her own brand of comfort to the men, despite Lenihan's orders that she stay away; and Terence O'Brien, a swaggering thug who accuses Kerry of cowardice for not committing himself to the Irish cause. He also briefly meets the charming Lady Fitzhugh, an elderly rebel sympathizer who plans to smuggle an injured Irish terrorist named Liam O'Sullivan to safety. The plan goes awry, and Lady Fitzhugh is arrested just outside a pub near rebel headquarters. When O'Brien's gun falls to the floor, the Black and Tans assume it belongs to Kerry, and he is dragged from the pub and imprisoned. Kerry is tortured by the Black and Tans, and by the time Lenihan and the others rescue him, he is ready to join the rebellion. During his recovery, Kerry deepens his friendship with Noonan and romances Kitty, who willingly accepts his attentions. When it is learned that Lady Fitzhugh has been on a hunger strike since her arrest, Lenihan suggests that they take a hostage who may be used to effect an exchange. Kerry assists in the kidnapping of Jennifer Curtis, the widowed daughter of military adviser Sir Arnold Fielding, but soon after her imprisonment in a nearby lighthouse, he finds himself falling in love with her. Lenihan reminds Kerry that there is no room for pity and mercy in their war, whereupon Kerry replies that no war is worth winning if those human qualities are forgotten. The discovery by the British of Lenihan's involvement in the kidnapping forces the Commandant underground, and when he meets with the General, he learns that British officials are considering a treaty that would guarantee dominion status for Ireland. Horrified that the General would approve of such a compromise, Lenihan swears to fight until all of Ireland is declared a fully independent republic. Later, Lenihan and the other men meet at the lighthouse to discuss their plan to go to Dublin, where they can ambush and kill Col. Smithson of the Black and Tans. Kitty's presence infuriates Lenihan, and he orders her into the back room with Jennifer. Later, Kitty sneaks out to the beach for a swim in the nude, and when she realizes Lenihan has been watching her, she dares him to touch her. Burning with hatred, Lenihan orders her to leave. Back at her pub, when Kitty discovers that the Black and Tans are looking for her, Donovan, her employer, gives her enough money to book passage to England. Meanwhile, Kerry, aware that the coming morning's ambush is fraught with danger, bids farewell to Jennifer, who by now returns his love. At dawn, the men take their places on the dock, ready to attack Col. Smithson's approaching vehicle. Meanwhile, as Kitty is about to board a vessel to Liverpool, she is detained by Black and Tans. Because of her surprise at seeing O'Brien on the dock, the guards become suspicious and chase him. A gun battle begins, during which Lenihan, assuming Kitty has betrayed them, ignores her pleas and shoots her repeatedly. Seeing this, Kerry, horrified at Lenihan's brutality, goes to the Dublin headquarters and hears the General announce that the proposed treaty has been approved, and that he is on his way to London. Following his departure, however, the remaining rebels learn that Lady Fitzhugh has starved to death. When Lenihan returns to the lighthouse, he hands Jennifer a bible and leads her to the shore, where he plans to carry out an execution of revenge. Kerry pursues them, reminding Lenihan of the treaty and shouting that the Commandant now kills solely for the sake of killing. As Lenihan aims at Jennifer, Kerry cries, "I'm not going to fight your war!" and shoots him. Following Lenihan's death, Kerry looks with pain at his gun and flings it into the sea.