The Mediator #2: Ninth Key by Meg Cabot is the second installment in her mediator series, whereby the main character, sixteen-year-old Susannah Simon, is forced to navigate the sometimes volatile waters of her teenage life as a spiritual leader of lost souls. Susannah must come to terms with her true identity as a mediator, while seeking some semblance of a normal teenage life. In this popular media coming of age series, the main character gradually crosses the line separating childhood from adulthood, much the same way as her lost souls cross the line between death and the afterlife.

The Ninth Key opens with Susannah meeting with the first and only other mediator she knows, Father Dominic, principal of her new high school. After parts of the high school are destroyed during one of Susannah's mediating experiences, Father Dominic admonishes Susannah for kicking her ghosts' butts to the other side. He would like to see Susannah take a more gentle approach with her spiritual charges.

Two spirits are introduced as they visit Susannah's bedroom. One is a mysterious hysterical woman who asks Susannah to tell Red that he did not kill her. The other is a handsome young ghost named Jesse. The next day at school Susannah finds out that Red is Tad Beaumont's father.

Susannah's father's ghost warns her to stay away from Red Beaumont. Stubborn and independent, Susannah cannot stay away. Susannah operates under false pretenses to get the chance to visit Mr. Red Beaumont. After telling him that she is doing a story for her school newspaper, Susannah visits the Beaumont mansion and delivers the dead woman's message. Susannah gets a bad feeling regarding the Beaumonts. Nevertheless, Susannah finds herself attracted to Tad. Tad takes Susannah out for coffee before driving her home after this first meeting with Mr. Red Beaumont. Susannah experiences her first kiss.

Susannah tells Father Dominic about her meeting with Mr. Red Beaumont; Father Dominic concludes that Mr. Beaumont is a vampire. Susannah thinks the idea ludicrous, but as events unfold, she begins to believe.

During dinner at the Beaumonts, Susannah makes observations, which feed into the vampire scenario. When Tad passes out after his father drugs him, Susannah panics. She stabs Mr. Beaumont in the chest with a wooden pencil. Marcus, Mr. Beaumont's assistant, tells Susannah that Mr. Beaumont suffers vampire delusions.

After some research, Susannah's friend CeeCee tells her that Marcus Beaumont, Mr. Beaumont's brother, began running the business after Mrs. Beaumont died. Before thinking about what she does, Susannah tells Tad about the events that transpired during dinner.

The mysterious female ghost returns and tells Susannah that she gave the wrong Red her message. Feeling depressed, Susannah dresses up in her best designer butt-kicking outfit to make herself feel better. The sister at school sends her home to change out of her inappropriate attire. As she walks home, Marcus Beaumont abducts her. Before Marcus gets the chance to kill Susannah, she finds a way to escape.

Back at home, Susannah sees a picture of her stepfather's first wife. She is the mysterious female ghost, a lady who died after the family made the decision to pull her off life support. Susannah tells her youngest stepbrother, Doc, or David, also known as Red, his mother's message.