Two young women are found dead on the Chicago lakefront in as many weeks, each murdered in the same fashion. Detective Sergeant Grayson MacCarthaigh is at her wit's end trying to solve the grisly mess before the murderer claims another victim. Her only common thread is Dr. Neala Rourke, the curator of the famous National Museum in Dublin, Ireland. She is in Chicago with her archaeological exhibit at the Field Museum, which suspiciously coincides with the two murders. With a strange and eerie turn of events, things start unraveling. Grayson now is compelled to return to her birthplace and follows Dr. Rourke back to Ireland where ancient Celtic beliefs and mythology are thrown into the mix, turning the detective's logical world upside-down. However, one fact remains for the determined Detective MacCarthaigh: Someone killed those two women and whether real or mythological - he or she will kill again.

In the sequel to Residual Moon, Grayson MacCarthaigh stays in Ireland, struggling with her destiny as the true descendant chosen to protect the ancient power of the fabled Tuatha De Danann. Her logical world is now replaced with Celtic legends and myths, goddesses, and immortals. As Grayson comes to grips with all this, someone or something is doing his or her best to see that she fails in her role of protector of the ancients. Is it her new archenemy, Phelan Tynan the ancient wizard, or Elinora, the beautiful but enigmatic immortal sent to help Grayson? And what of Sister Gabriel, the cloistered nun who suddenly appears at the abbey? Grayson soon realizes everyone has something to hide. Mendacity runs amok at St. Brigid's Monastery, but all will be revealed under the Liar's Moon.