In 1921 Dublin, the I.R.A. battles the "Black & Tans", special British forces given to harsh measures. Irish-American medical student Kerry O'Shea (Don Murray) hopes to stay aloof, but saving a wounded friend gets him outlawed, and inexorably drawn into the rebel organization under his former professor Dr. Sean Lenihan (James Cagney), who has "shaken hands with the devil" and begun to think of fighting as an end in itself. Complications arise when Kerry falls for a beautiful English hostage, and the British offer a peace treaty that is not enough to satisfy Lenihan.