Two thousand years before Cait Reagan was a twinkle in The Morrigan's eye, Weyna played handmaid to that very same Goddess in the year leading up to the Second Battle of Maighe Tuireadh, where the Tuatha de Dannan faced the forces of Fomori in a climactic final battle. But when Weyna stuns the assembled Gods and Goddesses by taking down the greatest fighter the Óșeni have ever known using a technique she never learned, she sets in motion a series of events that will ultimately lead to the discovery a long-buried secret two millennia later.
In the present, Cait's sister, Aoife Reagan, a former member of Ireland's Tacaíochta Faoi Arm, the Garda version of SWAT, is drawn to Boston for the funeral, only to find that it's not going to happen. Cait died, but it didn't take as Cait returned to the ranks of the undead somewhat posthumously. Unfortunately, Cait's soul has decided to go on walkabout, and she remains in a strange sort of stasis.
In the meantime, Liz believes something larger is at work, and she has concocted an insane plan to draw out Cait's killer and lead them one step closer to whoever is behind him. Aoife will impersonate Cait, leading the would-be assassin to believe they've failed. Now she just has to convince Aoife to be the bait, impersonate a nine-year veteran of the police department, and do it while chasing down a vicious creature of myth that is running loose in Boston and could destroy half the city.