Gemma Doyle is a regular old teenage girl walking around India with her mom and their maidservant. They are arguing because Gemma really wants to go to London, where all the cool kids are, but her mom doesn't care what the other kids are up to; she said no and she means it. Since Gemma is sixteen, this hard, cold no infuriates her, and she runs away, causing her mother to follow her down an alley.
Suddenly Gemma is struck with a strange vision of the future, including her mother killing herself and a demented shadow creature eating a guy (um… things officially just got weird, Shmoopsters). When she wakes up and tries to get back to her mom, she stumbles upon her vision come true. Yikes, right? Needless to say, it's super depressing and Gemma feels really guilty.
Later, Gemma's grandmother sends her off to Spence, a finishing school for girls, and Gemma gets a room with a poor student named Ann. Felicity and Pippa are the most popular girls in the upper class and they like to abuse Ann. One day, Felicity invites Ann to hang out, only to accuse her of stealing her ring (which she hid in Ann's knitting basket)—luckily for Ann, though, Gemma saves the day by figuring out Felicity's nasty plan and beats the mean girl at her own game.
The down side to all of this is that the cool clique, including Cecily and Elizabeth, invites Gemma to join their group… but only if she'll steal the holy wine from the chapel… and then when she tries to, they lock her inside. Gee thanks. Adding to Gemma's troubles is the fact that a strange man in a cape—named Kartik—follows her into the chapel, and then warns her not have visions any more, which, of course, she has no idea how to do.
While finding her way out of the chapel (with wine in hand), Gemma is visited by a spirit who leads her to a diary written by a girl named Mary, who writes about having visions and using magic with her friend Sarah. Interesting…
The new art teacher, Miss Moore, tells the girls about a group of witches called the Order. By chance, Gemma catches Felicity breaking a major rule, which she decides to hold over Felicity's head. So when Felicity gets the great idea to begin their own Order, Gemma says that Ann must be in too, and since Pippa does whatever Fee (that would be Felicity's nickname) does, the Order is made up of them four of them.
Even though they are all kind of getting along though, Gemma doesn't trust Felicity and Pippa, which is probably wise given how nasty they've been in the past. Then, one day during dance class, Pippa has a seizure. While the girls are sad about Pippa (she has epilepsy), they visit Mother Elena and, when they do, they discover that Sarah—Mary's friend (from the diary)—is wicked too. Really wicked.
You know who else is wicked? Felicity. And while we pretty much knew this already, once she's mean to Pippa it becomes crystal clear that this girl likes to have the power in a given situation.
During a séance, Gemma has a vision, and in it she meets with her mother, who instructs her to meet her in the garden and not to trust the Rakshana, whom Kartik works for. Gemma finally chooses to tell the other girls in the Order about her supernatural powers, and takes the group into the realms, which are beyond the human world and quite beautiful. Here Gemma gets to talk with her mother (yay) and learns about the Runes of the Oracle, where the magic is kept.
The girls go to the realms several times to play, but Gemma just can't accept that she is not skilled enough to handle taking magic out of the runes (her mother forbids her from doing so, citing immense danger from someone named Circe) and becomes obsessed with doing so. She might be kind of magical, but she's still a teenager, after all, and no teenager likes it when their parents tell them they can't handle things.
Each of the four girls gets wrapped up in their own drama, and since they're feeling so sorry for themselves, Gemma decides to steal a bit of magic from the runes. The girls use the magic to trick people at Spence, which is a good time and all, but also unleashes evil and corruption in the realms, which Gemma discovers when she talks to her mother… and finds out that she is Mary Dowd, the diary keeper, and her friend Sarah is actually the evil Circe. Oh, and Sarah is alive. Uh-oh.
Meanwhile, Gemma develops a major crush on Kartik, who it turns out is her protector. When Gemma senses that something in the realms isn't right, she keeps stalling about taking her friends back in—but Felicity gets angry.
Gemma gives in and takes all of the girls back into the realms, but they are attacked by the shadow creature that killed her mother and, while trying to escape, Pippa falls into the river and gets trapped under ice. Gemma saves the other two girls, but Felicity is all bent out of shape and blames Gemma for leaving Pippa behind to die.
Pippa is unconscious in the real world and Gemma goes back into the realms alone, faces her foe, forgives her mother, smashes the runes, and defeats the evil creature. At the end, she rescues Pippa from the water and wants to bring her home, but Pippa decides to stay in the realms forever instead of marrying the stodgy old man her parents have arranged for her to, and eats some berries (which makes it impossible for her to leave). Back at Spence, they have a funeral and mourn Pippa, and as the book ends, Kartik doesn't hate Gemma and Felicity might forgive her too.