It is the second book of Celtic-inspired middle-grade fiction by award-winning author Patti M. Walsh.

Thirteen-year-old Tony plays the harp, but wants to play soccer. His parents won't let him. While pining away for a chance to play-or even watch-a game, Tony runs into Ferdy, who convinces Tony to deliver drug money to a known criminal in exchange for game tickets. When he does, Tony is arrested. At the police station, the two meet again. "You set me up," Ferdy screams. "I'm gonna kill you!"

The threat hounds Tony and his parents. They have him use his middle name, Colin, and send him and his dog to live for the summer with his eccentric 70-year-old grandaunt Alba. Unbeknownst to Colin and his parents, she is a master of the Irish martial art bataireacht-shillelagh fighting. She emphasizes that he learn to defend himself by practicing discipline, visualization, and Celtic rituals. Colin resists the training. He'd rather play the harp. In a compromise, Colin teaches Alba to play the instrument.

Meanwhile, on a trip to the town of Tory Island, Colin meets Bonnie, who is living with her aunt Anam and uncle Nog. They are resident owners of Companion Moon, a bed-and-breakfast. Bonnie, a distant cousin, is the protagonist of Ghost Girl, the first book in the series. She is a blogger and videographer.

When he is not training and helping Alba, Colin enjoys spending time with Bonnie. He also meets the lifeguard James, who asks a lot of questions about his background.

Nog is a storyteller. One day, he tells the story of Cú Chulainn, the Irish warrior and his magic weapon. Colin is struck by the similarities between himself and the legend.

As Colin's self-discipline and bataireacht skills improve, Alba also teaches him about King Cormaic Mac Airt and his code of ethics. Colin discovers becoming a warrior is more than combat training.

Colin doesn't know that Alba has been having disturbing visions of him being attacked by a lizard-like character. Since she also dabbles in magic, she imbues Colin's shillelagh with a potion.

On a late summer day, while Colin and Bonnie spend a day in town, Ferdy shows up. After being released from juvenile detention, he has come to seek revenge. He found Colin because James reposted a picture of Colin on Bonnie's blog.

Ferdy challenges Colin to a fight at dawn. Colin tells Bonnie that he has been learning bataireacht, and she tells him that she not only sees ghosts but has learned from them that she is responsible for everyone in her family, including him. She slips out in the middle of the night to witness the battle.

Alba also shows up, for her instructions and magic are being put to the test.

Ferdy is expecting the timid Tony. Instead, Colin, the Celtic warrior, shows up. Following an intense showdown, Colin defeats Ferdy, not by physically injuring him but by overcoming him with his magic weapon. Colin forces Ferdy to return home and acknowledge his guilt.

When Colin's parents and Bonnie's parents arrive to bring their children home, Alba hosts a party. Not only does Colin play the harp, but Alba does, too. Colin and Bonnie look forward to spending the next summer together.