Darkest Hour is a novel by author Meg Cabot and is fourth in the series of the "The Mediator" novels. In this novel, Susannah, is a 16 year-old girl that is also a mediator. As a mediator, Susannah talks with ghosts to help them resolve the issues they had when they were alive so they can pass on to their next life. Susannah takes readers into the lives of her family and the other people living in the town of Carmel, California while she works as a babysitter at the local hotel resort.

As the story unfolds, the readers learn about the many situations Susannah has found herself in with ghosts in the past. Susannah's encounters also help to unravel the secret disappearance of the 20-year-old Spanish male ghost that haunts Susannah's room. When her stepbrother and stepfather dig up his skeleton while digging the hole for the family hot tub, Susannah realizes that Jesse was in fact murdered by his fiancé and her boyfriend. To make matters worse, the dead fiancé and her husband, Felix Diego, ghosts show up to try to kill Susannah.

In the meantime, one of her charges at the resort is an eight-year-old boy that Susannah realizes is a mediator as well. While mentoring him, the two form a special relationship that leads to Jack performing an exorcism to get rid of Jesse, who Maria convinces Jack is harassing Suze. Suze convinces Father Dominic, the school priest, principal, and fellow mediator, to exorcise her to the afterworld so she can bring Jesse back to Earth.

In the end, Susannah is able to keep the secret from her friends and family members that she is a mediator. In the meantime, she is able stop her own murder and help Jesse return to living in her room as a ghost.