Note: Wikipedia contains spoilers
Two interesting notes- This novel preceded the "scare" of AIDS, Ebola, and SARS by many years, in envisioning a global pandemic disease. Additionally, the ease in which O'Neill designs the plague he unleashes foresees the future in which diseases can be custom designed and tailored for a specific population (female, in this case).
Typical of a Herbert novel, most of the drama is mental, political, and communal. While the story of O'Neill and his revenge is told in full, there is also the larger story of how the governments of the world's countries deal with the plague, which escapes into limited areas that are quickly sterilized by "panic fire", which immolates everything and everyone in it. North Africa is wiped out; Boston is burned to the ground; Rome is destroyed with atomic bombs; and the U.S. pushes for a moat of cobalt dust to isolate Africa, which is written off as a total loss.
The world's armed forces are reorganized under a Canadian admiral, Francois Delacourt, who heads Barrier Command, responsible for the absolute separation of contaminated and clean areas. Scientists toy with a conspiracy of intellectuals to override the expected repression of research by governments. In countries around the world, angry mobs lynch Irish, English, and Libyans, and anyone too closely resembling them.
O'Neill is driven halfway insane by the death of his family, and his mind fragments into several personalities that carry out his plan for him. After releasing the plague, he goes to Ireland to hide, planning to offer his services as a molecular biologist in the hopes of sabotaging whatever work is done there on finding a cure. When he arrives in Ireland, he is suspected of being O'Neill (whom the investigatory agencies of the world have deduced is responsible). To travel to the lab at Killaloe, he is forced to walk with a priest, a boy struck mute by the death of his mother, and Joseph Herrity, the IRA bomber who detonated the explosive that killed O'Neill's wife and children; their purpose is to confirm his identity, either through Herrity's indirect questioning, or the possibility that he will confess to the priest when confronted with the pain his revenge has caused for the boy.
Meanwhile, law and order have broken down in England and Ireland, and the old Irish ways are coming back. Local IRA thugs appoint themselves 'kings of old', and others recreate old, pagan religions centred on the rowan tree. The IRA has effective control of Ireland, but as the governments of the world grow certain that O'Neill is there and in effective custody, they start to consider that wiping out the three targeted countries with panic fire might be the most effective end to the lingering threat.