After her father is gone to serve the military and her grandmother Frankie is bound to bed because of illness, Evie is forced to leave her seaside life behind and join the Abbey boarding school in Wyldcliffe. There are gossips about the place, about its haunted reputation. On her way there, Evie runs into a mysterious boy on a horse, and is left intrigued by the encounter.
At the school, Evie is greeted by the headmistress who immediately sets up strict rules for her, considering that she is there on a scholarship, unlike other students, except Helen. One of those rules is not wearing any jewelry, like the necklace that Frankie gave to her and that Evie has around her neck at that time.
Evie is having a hard time accustoming to this new life. One of the students called Celeste keeps targeting her for taking the place at school of her previous friend, who drowned at the lake. There is another thing that makes it difficult for Evie; she keeps having visions of a girl that peculiarly resembles her. The plot of the novel is split into two: the present time with Evie and the past told through the journal of Agnes, a daughter of the previous owner of the Abbey. The plots are strangely intertwined, with events that mirror each other.
Evie makes a friend at Abbey, a nature-loving girl called Sarah. She also starts sneaking out of the school almost every night to see the mysterious boy from the first night. His name is Sebastian, and Evie catches herself falling for the strange dark-haired boy.
All the while this is happening in the present, we get a glimpse of the Wyldcliffe's past. Agnes had a friend whom she refers to as S. in the journal. S. brought a strange book from one of his travels which he believes will give both of them incredible powers. He persuades Agnes to start using the book and the book enables Agnes to reach elemental powers of fire, earth, air and water, which is beyond human comprehension. Agnes uses the spells from books to heal and help people around her and S. grows more and more jealous by the day. He persuades Agnes to give him the access to powers as well, but, he is nowhere near her. Agnes realizes that S. is falling to the dark side as the days go by and his hunger for more power grows.
Sarah is worried about Evie, and she reveals to her that she keeps a secret, a power to sense the future and danger, she senses that there is something wrong with Evie and tries to convince that the visions she's been having are not coincidental, or a part of her imagination. Together they set out to find out more about this and discover that the girl from Evie's visions is Agnes and that she is her ancestor. She also finds a picture from a boy that was connected to Agnes at that time, a boy named Sebastian, and he looks exactly like Evie's Sebastian.
Evie decides to meet with Sebastian and demand truth. He tells her that he is that same Sebastian, that he's done some bad things which led to his immortality. He gives her Agnes's journal and Evie reads the rest of the story. Agnes was scared of Sebastian and his demands to grant him immortality, so she fled to London where she met, loved, married and lost a man with whom she had a daughter. Thinking of her daughter's future she returns to Wyldcliffe with her and tries to find courage to go to her parents. Before she does that, Agnes casts a spell that makes all of her powers be locked in a pendant necklace, so that Sebastian can't use her powers for himself. One night she leaves her daughter and the necklace with her old nanny and decides to go to her parents.
Sebastian tells Evie the rest of this story. He found out that Agnes came back, and he ambushed her on that night demanding of her the gift of immortality. Furious that she refuses to give it to him, he pushes her, and she hits her head on a rock and dies. He didn't know about her daughter or the necklace which was safely hidden for generations.
In the meantime, Sebastian found a way to outlast normal life, but with the cost of living in the shadows, neither alive nor dead, until he met Evie who made him feel alive again. But, now they have to keep away from each other, because in order to get out of the world of shadows, Sebastian has to take the necklace by force from Evie, he has to kill her. He is afraid that he might hurt her the more he begins to fade and hunger for life becomes stronger, so he tells her to leave.
Back at Abbey, Evie together with Sarah discover that the teachers at school, led by the Headmistress Hartle, are all part of a coven which works on making Sebastian truly immortal, which will make them like that as well. Helen is unwillingly a part of that as well, because she the Headmistress's daughter. Helen, Evie and Sarah decide to join forces to make Evie awaken her powers from the necklace and fight against the coven.
With the help of Celeste's prank, Evie gets detention and is forced to go to the headmistress. The girls also discovered that detention is just a disguise, and that the girls who go to detention are sacrificed to prolong Sebastian's life. Evie manages to escape to the tombs, but that is only a trap. The coven gathers and brings out Sebastian to kill Evie, he refuse because he loves her. Evie finally awakens her powers and defeats the coven, which disappears together with Sebastian. Evie decides to try to find a way to fight for love and find a way to rescue both herself and Sebastian.
