Cassie Blake's world is turned upside down after her mother dies in a mysterious accident, forcing Cassie to move in with her loving grandmother Jane in the small town of Chance Harbor, Washington. While trying to adjust to her new life, Cassie is quickly befriended by Diana Meade, a sweet-natured classmate who offers to show her around and introduces her to mean-girl Faye, her sidekick Melissa and Cassie's next door neighbor Nick. Things get complicated when she meets Adam, Diana's boyfriend, with whom she feels an instant and powerful connection. Upon meeting one of her mother's childhood friends, Dawn Chamberlain, the school principal and Faye's mother, Cassie begins to wonder why her mother never spoke of her hometown. When strange and dangerous things begin to happen, Cassie's new friends are forced to tell her their secret - they are all witches and her arrival will complete the Circle. However, the Circle has not been drawn together again by chance - Diana's father, Charles Meade has plans of his own for the group.

In an effort to distance herself from the Circle and establish a normal teenage life in Chance Harbor, Cassie befriends Sally Matthews, a non-witch. Concerned their powers are getting out of control, Diana pushes everyone to bind the Circle, but Faye is thrilled with the increase in her powers and refuses to participate, while Melissa flirts with Nick despite Faye's warnings. Meanwhile, Adam fights his growing feelings for Cassie, but circumstances continue to throw them together, forcing Cassie to make a decision.

A cute jock named Luke asks Cassie to the school dance, and after watching Adam spend time with Diana at the boathouse, she accepts his offer. Meanwhile, Melissa sees the dance as an opportunity to get closer to Nick. Faye, on the other hand, has no plans to go and remains focused on finding a way to control her powers. Dawn discovers someone from their past, an intense man named Zachary, is in town asking questions about the Cassie and the Circle, so she asks Charles to keep an eye on him.

Cassie looks up an old friend of her mother's named Heather Barnes and discovers she has been in a catatonic state since the night of the fire 16 years ago. Feeling guilty that her mother and her Circle may have had something to do with Heather's current state, Cassie tells Diana that she wants to use magic to help Heather. Diana refuses to help, but Faye is more than happy to conjure up a spell - for a price.

Torn between her loyalties to the Circle and her family, Cassie wrestles with the secrets she must keep from her grandmother. Nick is thrown for a loop when Melissa adopts a new attitude and enlists his help to track down her family's Book of Shadows. Faye is also recruited but isn’t happy about being the third wheel. Meanwhile, Diana is craving some alone time with Adam and arranges a date night so they can rekindle the spark in their relationship, but all six of them are brought together to fight off a dark force that threatens to destroy the Circle from within. Completely unaware of the new danger encroaching on their children, Dawn joins Charles aboard his boat to further their plans for the Circle, but is surprised to find that Charles may have more than business on his mind.

When a crystal is magically stolen from the abandoned house, the Circle decides it was the traitor witch working with Eben so Jake calls for a meeting with Isaac to see if they can get him to change sides. Cassie’s loose grasp on her dark magic threatens to destroy any possible alliance when their pursuit of the traitor leads them to the “Creepiest Place on Earth.” Meanwhile, Faye and Jake work together to steal Dawn’s crystal, Melissa and Adam pair up and uncover a new magic trick. Meanwhile, a frustrated Diana reaches out to Charles for help.

It’s Prom night in Chance Harbor and after Adam uncloaks a crystal hidden in the school, Blackwell tells Cassie she can use her dark magic to find it. When Cassie follows this advice, it leads her to a dangerous discovery. Meanwhile, Faye invites Jake to Prom, despite the fact that he stood her up two years earlier. Adam, Diana, Cassie and Melissa finally make it to Prom but when they lose the crystal to Eben, they end up in a life and death situation. Meanwhile, Dawn attempts to stop Blackwell, who casts a deadly spell on Charles.