Timebound opens with the protagonist, Kate, going with her mother, Deborah, to meet her grandmother, Katherine. Kate’s mother has a very strained relationship with Katherine because she feels like Katherine wasn’t there for her when she lost her father and her twin sister, Prudence. When they arrive, Katherine tells Kate and Deborah that she’s dying and only has a year left to live. With the help of Kate’s father, Harry, Katherine was able to find a nearby place to live. She wants Kate and one of her parents to move in so they can spend time together. While they are talking, Kate notices a strange blue glowing medallion in Katherine’s possession, but nobody else seems to notice it. When Deborah goes to the bathroom, Katherine talks to Kate about panic attacks she was having earlier in the year and how they’re normal.

Kate and her father agree to move in with Katherine and her assistant, Connor. Katherine gets a moment alone with Kate and explains that the medallion is actually a CHORNOS device which allows people with a specific gene to travel through time. Kate has inherited that gene. She gives Kate a diary to read and says that it will explain things further, and they’ll speak more about it later. Kate is overwhelmed and tells her best friend, Charlayne about what happened. The two read through the diary together, which turns out to be a very advanced computer. The diary details Katherine’s journeys in the past.

The next day Kate is attacked on the train by someone named Simon. She’s rescued by a man named Kiernan; someone that Kate saw in what she thought was a hallucination earlier when she touched the CHRONOS medallion. Simon gets away with Kate’s bag, which contains the diary. Kiernan makes it clear that he and Kate knew each other in a different life and loved one another. He tells Kate that she’s in danger and she needs to get to Katherine immediately.

Kate manages to get to Katherine who explains more about the medallion. Katherine used to be married to a man named Saul. Like her, Saul was a historian, someone who travelled in time to personally witness and record what happened. Katherine noticed that Saul was going against regulations. He started to take advanced technology with him on their jumps, and Katherine later learned that he was preaching under the name of Brother Cyrus, using his knowledge of the future to make prophecies.

When Katherine discovered that Saul was actively preaching and changing history she decided she had to say something. The same day that she was going to report him, Saul attacked and destroyed the CHRONOS headquarters. Katherine was able to escape, but whenever she tried to use the medallion it didn’t work for her. Anyone who inherited the CHRONOS gene was able to use the medallion. Katherine found this out because Prudence was able to use the medallion. Prudence snuck the medallion out and used it to jump, which is what caused the accident that killed Deborah’s father. Katherine believes that Prudence is now working with Saul.

Kate wants to tell her father everything that Katherine just told her. Katherine hesitantly agrees. Kate goes to see him, but he isn’t in his office. On her way there, she has another panic attack, which Katherine explained is a result of the timeline changing. Kate finds a strange teacher in her father’s classroom, and there’s no record of him working in the school. A student Kate has never seen before, Trey, is ordered to take her to the nurse. Kate is distressed and tells Trey everything. She shows Trey a picture of her mother, which vanishes the moment she puts it on the table because it isn’t protected by the CHRONOS. This convinces Trey she’s telling the truth. They try to find her father, but he’s in a relationship with someone else and has no idea who Kate is.

Katherine tells Kate that Saul must have killed her in the past. Because of that, the timeline was changed. Deborah was never born, so Harry never got married and had Kate. Kate and Katherine were protected because they have CHRONOS devices, but if they lose the device they’ll disappear. Kate and Trey develop a relationship while Katherine teaches Kate how to use the device. Katherine and Connor also explain to Kate how Saul created the Cyrist religion. In Kate’s timeline they were a minor religion, but after the time change they’ve become a huge organization with a huge amount of influence in the world. Even the president is a Cyrist. Katherine also warns Kate that if she fixes the timeline and gets her parents back, she’ll lose Trey.

Kate sneaks out one morning to go to the Cyrist church because she saw that Charlayne was a member. While there, she gets discovered but manages to steal the book of prophecy, a holy Cyrist book that only elite members are allowed to read from. Later, Kate is attacked by Simon again. Katherine comes out to save her, but in exchange she has to give up her medallion, so she vanishes from the timeline. Kate jumps back in time to Chicago, to the day that Katherine died.

Kate meets a young Kiernan who helps her track down Saul and Katherine. She also runs into Prudence, who reveals that she’s trying to save Katherine. She’s protected by her CHRONOS device, but since Katherine died it means Prudence doesn’t exist either. She doesn’t enjoy Saul having that much power over her. She truly believes in the Cyrist way, but she wants to lead them instead of Saul. An older Kiernan travels back to help his younger self and Kate to save Katherine.

Kiernan reveals that he and Kate used to be part of a resistance movement against the Cyrist church. Kate disappeared one day when the timelines changed. He has feelings for this version of Kate, but he knows she isn’t the same person, and she has Trey. Kate returns to the present and both Katherine and her parents are restored. Saul is still out there, but Kate is determined to help Katherine stop him from ruling the world through the Cyrist religion. Kate tells her father everything, and at the end of the book she gives Trey a DVD that he made which contained all of their videos together. She tells him to get in touch with her after watching the DVD, confident that they’ll be able to get back together.