The title refers to the Pax Hibernia, in effect since about 1500, when Ireland conquered the Spanish empire, and became the de facto ruler of Hibernia (Ortho earth). Additionally, it is the title of a novel within a novel, consisting of portions of plot #2 that are referred to by characters in plot #1.

There are three interwoven plots.

Plot #1 (starting on Tirdia (Earth Prime) in the summer of 1945)

Sally O'Neill and Lucy O'Brien are successful secret agents for Ireland but struggle with the morality of their profession and their own spirituality. They arrive in Calgary during the dying days of World War II to buy back stolen atomic secrets for the Allies. They encounter and rescue a captured officer serving another Ireland (on Ortho Earth or Hibernia) and are recruited to his cause when they learn his enemies plan to frame the Japan of his world with the stolen secrets as an excuse for mass destruction.

Travelling to Hibernia, they unravel the plot against their friend and his three royal cousins, marry two of the men, and join the ongoing war between the banished royals and the MacCarthy families, who run Hibernia as a corrupt protectorate.

The sequence culminates in a showdown over yet another import of banned weapons aimed at destabilizing Ireland's Peace.

Plot # 2 (interwoven, from 1941, as a novel within the novel)

James IV, most Christian High King of Ireland, is deposed by his nobles, anxious over the slow progress of their version of the Three Worlds' War. Banned from Tara's throne sixty years, he and three cousins clandestinely promote traditional values. They spike the high (atomic) armory, and steal MacCarthy secrets to stop a genetically engineered plague, reasoning it better to fight honourably than to win a short but shamefully waged war.

Exiled to thinly populated Irish North America, they find romance, decode hostile nanomachines, administer justice, and fend off schemes to re-ignite the war. Chief Walking Buffalo's remote capital of Edwardston becomes the struggle's centre as the racist MacCarthys scheme to commit mass murder, and the royals deploy new technologies to prevent Greater Ireland's collapse and preserve their own line for a claim on the throne in 2001.

Plot #3 (interwoven, starting in 1978 on Ireland's home island)

Brian McIlhargey, ostensibly a soldier of the defeated Devereaux army awakens in hospital to find himself lacking an arm and with a reconstructed face. Allowed to leave, he finds the family manse burned, and fresh graves for Katherina Rourke, Jack Devereaux, and their infant daughter Mara. Vowing revenge for the atrocity, he soon encounters a soldier attempting to rape household servant Karen O'Toole.

Rescuing her, he discovers she has extracted baby Meghan from the inferno. He adopts both, taking them into exile as a mercenary specializing in electronic security and the sword. Scenes over the years in both tiny Detroit and metropolitan New Tara suggest that Brian is actually the son of a royal cousin from plot #2, and Meghan is also far more than she appears to be.

Following a confrontation with a young hothead who kills Karen, the pair settle in Edwardston, where they have a close call with a false priest, whom Meghan kills in a sword fight, and we discover Brian is turning her into a potent revenge weapon. This subplot ends with an oath to that end over the graves of characters from plot #1, merging the sequences and concluding the story cycle. An epilogue promises more in subsequent volumes.