From 1649-50, Oliver Cromwell, the Protestant Lord Protector of England, waged a brutal campaign of total war against the Catholic and Royalist citizenry of Ireland, remaining a polarizing cultural figure in both countries centuries later. In part one, Cromwell's initial attacks against the Catholic and Royalist forces are successful, resulting in atrocities at Drogheda and Wexford and the death of opposition leader Owen Roe O'Neill.
In part two, Cromwell's army unexpectedly begins to founder as he fails to take two key strategic positions at Duncannon and Waterford. Then, in the driving rain of Kilmacthomas, his army reaches a low point when it is reduced by disease and slowed by the foul weather.