Ysabel follows the story of Ned Marriner, a 15 year old whose father is working in France, who along with his partner, Kate Wenger, an American exchange student, finds himself caught up as a character in a story that goes back a long, long time. There is a herald character who helps to incite the journey, a strange man who finds the two teens at the Saint-Sauveur Cathedral and warns them to leave.
Ned's father is a professional photographer who is working on a book of photos of France, so while he's considering the strange interaction at the church, he goes along with his father's team to help get them pictures, but he becomes sick. Suddenly, at Mont Saint-Victoire, he has a vision of the massacre that occurred there when the Roman army killed a Celtic community there. When they leave the mountain, Ned returns to normal.
The young couple reconvene and discuss Ned's strange abilities. They agree to meet again in Entremont, a relevant historical location to the massacre in question. Ned's abilities allow him to detect when the strange man from the cathedral is nearby, and suddenly he realizes that he's being watched by the man. He confronts the man who attempts to leave but is attacked by abnormally angry dogs, and Ned saves him from the dogs.
Ned needs a helper, so he goes to his Aunt Kim, a magician of sorts. He tells her about his abilities, and she indicates that the abilities are hereditary and psychic in nature. Then, a large Celt with antlers confronts them and warns them to stay out of whatever they're starting to involve themselves in. The Celt says that the stranger is a Roman and so the Celt threatens Ned for helping the enemy.
The couple then reconvenes according to plan, dodging a cult of Druids in a ritual procession. Kate finds herself hypnotized by the Druids and wants to join, but before they can escape, Melanie, one of Ned's dad's companions, finds Ned. Melanie then becomes a vessel for a new character named Ysabel who is a spirit and possesses Melanie's body. She then explains that the two men, the Roman and the Celt are actually spirits of fallen humans in a timeless battle for the love of Ysabel.
The kids then decide to collaborate with the adults to help bring Melanie back from Ysabel's possession. They find Uncle Dave to be especially helpful, Kim's husband and a psychic as well. Following a portent, Ned decides to return to the mountain that made him ill. Ned reaches Ysabel first, and the two suitors commit suicide. Ysabel departs from the body of Melanie, leaving her unharmed.
