When Lisa Morton, author of The Halloween Encyclopedia, is called in to consult on the recent discovery of a fifteen-hundred-year-old Celtic manuscript, she's at first excited about the light this monumental find might shed on Samhain, the mysterious Celtic precursor to Halloween. Conor ó Cuinn, the Irish archaeologist who excavated the manuscript, thinks it reveals ancient magic. Lisa is skeptical...until people around her begin dying. Dr. Wilson Armitage, the university professor who was translating the manuscript, is found torn apart by wild animals...or was he actually attacked by vicious sidh, malicious Celtic spirits that wreak havoc every Samhain? As October 31st approaches, the border between our realm and one of murderous spirits begins to dissolve. Can Lisa survive Halloween night and use her knowledge to set the world right again?