A young man and woman are chased by a group of hooded men intent on killing them and taking a device that they carry. As they are surrounded, the man Paul reassures his companion that Gwendolyn will be safe, that Grace will protect her. Just as the men are about to kill them, Paul and the young woman activate the device and disappear.
The film fastforwards sixteen years later, to the day before Gwendolyn "Gwen" Shepherd's sixteenth birthday. She belongs to an aristocratic London family who do not believe in associating with "commoners". They look down on Gwen because she, her mother Grace, and her brother Nick are the only "normal" family members. The family gathers for a party in honor of Gwen's cousin Charlotte, who is one day older than her. They hold a special regard for Charlotte as she possesses a secret genetic mutation, inherited by only one female per generation, that gives her the ability to travel back in time. At the party, Charlotte taunts Gwen about being unable to get a date for the school ball. Gwen is attracted to Gideon de Villiers, but he only has eyes for Charlotte. When Charlotte suddenly becomes ill, Gideon rushes her to a mysterious secret society, the Order of Count Saint Germain; he apparently is a carrier of the same gene. Believing Charlotte to be the "Ruby", the long-prophesied last time traveler, the Order inducts her.
Shortly afterwards, Gwen also becomes ill and begins seeing ghosts. She goes out to get some fresh air and stumbles into the nineteenth century. The only person she tells about this time jump is her best friend Leslie, who quickly recognizes that Gwen must be the one who inherited the time travel gene instead of Charlotte. Gwen makes several more uncontrolled jumps in time and learns about a conflict between her family and the Order. Gwen finally tells her mother about her time traveling. Grace recognizes that Gwen is the Ruby and brings her to the Order for help controlling her ability. Gwen's aunt, Charlotte's mother, does not believe her and accuses her of attempting to be the center of attention. The Order's guards fear a conspiracy, since decades ago Grace helped the previous time travelers, Lucy and Paul, escape. Gideon, also known as Diamond, does not want to work with Gwen because of his closeness to Charlotte—they have been training together for years for a mysterious "mission"—who in turn is upset about being replaced by Gwen.
Gwen is eventually confirmed as the so-called "Ruby", the last of twelve time travelers, but has doubts about the Order, the mission, and the Chronograph, a device the Order can use to control the time traveling of the twelve travelers. Lucy previously helped steal the Chronograph because she was afraid of a terrible power it held. Gwen still is unsure what exactly is going on, but at the Order, she is given an unlimited spending account, and Gideon is warned to keep an eye on Gwendolyn, and is also warned against any love affairs between the two time traveling families. Gwen overhears him say that she is definitely not his type.