Ireland, A.D. 671. An Anglo-Saxon delegation arrives in Cashel to debate the new religious rules that have been handed down from Rome. The Abbot of Imleach leads the Irish delegation, which is hostile to the new rules from outsiders. Among the Anglo-Saxon group is Brother Eadulf's own younger brother, Egric, whom Eadulf hasn't seen for many years.

When the debate quickly becomes acrimonious, a local abbess has to step in as a mediator between the two sides. But not even a day later her body is discovered, bludgeoned to death. The Chief Brehon Aillín accuses young Egric of murder, and suspicions and tempers run high. With the war of words threatening to spill over into bloodshed, Fidelma is sure there is something more sinister behind the murder than religious differences, and she is resolved to find out what really happened-and why.

Winter, 670 AD. King Colgú has invited the leading nobles and chieftains of his kingdom to a feast day. Fidelma and her companion Eadulf are finally home for an extended stay, and have promised their son, Alchú, that they'll be able to spend some time together after months of being on the road, investigating crimes. Fidelma and Eadulf are enjoying the feast when it is interrupted by the entrance of a religieux, who claims he has an important message for the King. He approaches the throne and shouts 'Remember Liamuin!' and then stabs King Colgú. The assassin is slain, but does enough damage to take out Colgú's bodyguard, and to put the king himself on the verge of death.

As King Colgú lies in recovery, Fidelma, Eadulf, and bodyguard Gormán are tasked with discovering who is behind the assassination attempt, and who Liamuin is. They must journey into the territory of their arch-enemies, the Uí Fidgente, to uncover the secrets in the Abbey of Mungairit, and then venture into the threatening mountain territory ruled by a godless tyrant. Danger and violence are their constant companions until the final devastating revelation.

Atonement of Blood is a mystery of Ancient Ireland from Peter Tremayne.