Rachel Crane was just six when her older sister, Brooke, died in a car accident ten years ago at the age of sixteen. Rachel often wishes that Brooke was still alive and there for her. That fantasy has to take a back seat to the reality of being sent to stay with her grandmother for the summer, leaving Rachel's boyfriend Duncan behind in Lake Melrose, Oregon.
Just before she's supposed to leave home, Rachel runs up to the attic--a place she often goes for some down time. A mysterious antique musical clockwatch literally appears at her feet. She picks it up, feels slightly dizzy, and finds herself transported back in time to the day her sister died.
Before Rachel can prevent the tragedy from happening, she must convince Brooke and herself that she is not losing her mind or caught up in a dream. After saving Brooke from her fate, Rachel finds herself back in the future, where it appears the whole thing was merely a dream when she suffered a mild concussion.
Then Rachel realizes she really did travel back in time and, as a result, changed her sister's destiny to something even more ominous. Brooke is now killed one week later by an unknown perpetrator. While trying to understand the mysterious clockwatch and its origin, Rachel uses it to go back again to try and save Brooke.
Through the power of the watch, both girls end up in the present as sixteen-year olds, with more changes as a consequence. Rachel's father does not recognize Brooke as having ever existed. However, strangely enough, her Nana does recognize Brooke, but that is dismissed by others due to her age and the early stages of senility.
Rachel has an ally in her boyfriend Duncan, a science whiz, who believes her incredible story and wants to help any way he can. But first they must discover the origins of the clockwatch and the Sisters of Time it belongs to.
Meanwhile, Rachel, Brooke, and Duncan face danger when the serial killer who tried to kill Brooke ten years ago has targeted her again in the present, along with Rachel. The girls must identify him before he succeeds with his deadly plans, even as they also grapple with any other possibilities the clockwatch may have in store for them in the passages of time.