During WWI Irish-American Army officer Trevor Sullivan is wounded on the Western Front serving with the famous Lost Battalion. At the end of the war he marries an Irish woman studying art in Paris. Both their families have a history of Irish rebellion against British rule. By 1920, Ireland is again in the throes of armed rebellion. Unlike the Easter Rising of 1916, this time the Irish population supports independence.
Sullivan's first visit to Ireland ends in personal tragedy drawing him into the conflict fueled by revenge and hereditary resentment to British rule. Experiencing the oppression of ethnic Catholics in Ulster by the Protestant loyalist majority supported by British Crown security forces provokes him to offer his services as a spy for the IRA.
A rogue intelligence unit of the Royal Irish Constabulary emerges as the central enemy in the North of Ireland. Reinforced by a new auxiliary police force, the RIC launches a campaign to crush the Belfast IRA through terror using torture and extra-judicial murder. Sullivan embarks on a singular mission to help the beleaguered republican cause.