Irish-American Kerry O'Shea is studying at the College of Surgeons in 1921 Dublin, Ireland, during a guerrilla war – the Irish War of Independence. Apolitical and sick of killing after fighting in World War I, he is drawn into the struggle between the Irish Republican Army and the British Black and Tans. He and his friend and fellow medical student, Paddy Nolan, are caught in the middle of an IRA ambush, and Nolan is shot by the British. Nolan tells O'Shea to fetch Sean Lenihan, one of their professors. Lenihan, it turns out, is also a high-ranking IRA leader known as "the Commandant". Lenihan removes the bullet, but Nolan dies anyway. Since O'Shea left his textbook at the scene of the ambush, he is now a wanted man. Lenihan takes him to meet his superior, "the General", an old comrade-in-arms of O'Shea's father. When O'Shea refuses his invitation to join the IRA, the General arranges for a boat passage out of Ireland. Lenihan takes him to a hideout by the sea, the base of an IRA unit commanded by Chris Noonan. Lenihan is furious to find local barmaid Kitty Brady consorting with the men there. When Liam O'Sullivan, a top IRA leader, is wounded escaping from prison, O'Shea agrees to accompany the unit to the rendezvous point to treat him. O'Sullivan is discovered in the boot of the car of aged Lady Fitzhugh and killed in a shootout by the British. When the soldiers check the people in the nearby pub, Terence O'Brien tries to hide a pistol he brought. When it is found, it is O'Shea who is taken away. He is brutally beaten by Colonel Smithson of the Black and Tans, but refuses to talk. Lenihan leads a raid to rescue him. At that point, O'Shea decides to join the IRA. Lady Fitzhugh is sentenced to prison and goes on a hunger strike. Lenihan kidnaps Jennifer Curtis, the widowed daughter of a top British adviser, to try to force a prisoner exchange. Complications ensue when Kerry falls for her. When Kitty gets into trouble, both with Lenihan and the British, she decides to leave Ireland. Lenihan prepares to assassinate Colonel Smithson at the dock. However, he suspects he has been betrayed when Kitty, purely by coincidence, tries to board a ship there. During the ensuing shootout, Lenihan shoots Kitty dead in cold blood. When the men reassemble at a lighthouse, they hear two bits of news. First, Lady Fitzhugh has died. Second, the British have offered a peace treaty. The General is satisfied to have peace, but not Lenihan. When he decides to execute Mrs. Curtis, O'Shea has to stop him. They exchange shots, and Lenihan is killed.
