The first book in the Eighth Day series follows twelve-year-old Jax Aubrey, an orphan living unhappily with his young guardian, Riley Pendare, in a run-down house in western Pennsylvania. Jax's mother died when he was young, and his father, Rayne Aubrey, was recently killed when his car was forced off a road into a river. After the accident, Jax's mother's cousin Naomi took him in, but Riley, an eighteen-year-old mechanic, arrived with a guardianship affidavit signed by Rayne three weeks before his death. Despite resistance, Naomi and a family lawyer inexplicably dropped their objections after private conversations with Riley, and Jax was taken to live with a neglectful stranger.
The morning after his thirteenth birthday, Jax wakes to find the world apparently empty: no electricity, a strange pink-purple sky, and not a single person anywhere. Convinced he is the last human alive, he breaks into a Walmart for supplies and spends a terrified day. The next morning, everything returns to normal, but the date is Thursday, skipping over a mysterious empty day between Wednesday and Thursday.
The following week, the same thing happens, but this time Riley is present. Riley explains that the extra day, nicknamed "Grunsday," is an eighth day hidden between Wednesday and Thursday. Jax and Riley are "Transitioners," descendants of Welsh clan leaders who experience all eight days. "Normals," the rest of humanity, live only in the seven-day week. A third group, the "Kin," is an ancient race who live only on Grunsday, unable to experience the other seven days. Riley points out a pale-haired girl in their elderly neighbor Mrs. Unger's upstairs window, explaining that Mrs. Unger's supposed ghost is a Kin person living there unseen.
Riley gives Jax strict rules: tell no one about the girl, and leave her alone. The narrative shifts to the girl's perspective, introducing her as Evangeline. She descends from the sorcerer Merlin Emrys, whose bloodline created and sustains the Eighth-Day Spell, the enchantment maintaining Grunsday. Evangeline has lived in Mrs. Unger's house for five years of her time, equal to thirty-five Normal years. The Taliesin clan placed her there after her parents were killed, separating her from her siblings Elliot and Adelina. At eleven, when Evangeline affirmed her father's goal of breaking the spell, one Taliesin struck her and called her nothing but a vessel for the bloodline. She has been alone ever since.
A.J. Crandall, Riley's frequent visitor, tattoos the Aubrey family crest onto Jax's wrist, a ritual marking that enhances magical potential and activates inherited abilities called "bloodline magic." Jax soon discovers his talent, "inquisition," which compels people to answer his questions, while Riley demonstrates his own "voice of command," which physically compels obedience.
Riley introduces Jax to his sworn vassals in a feudal system traditional among Transitioners. Mr. Arnold Crandall explains the origins of the eighth day: Around two thousand years ago, powerful Kin clans made war on Normals. Niviane, a Britannic queen, conceived the plan; Merlin cast the spell, entering imprisonment with his race; and Arthur Pendragon and a dozen clan lords contributed their magic. Another vassal, Melinda Farrow, begins training Jax. Melinda, a "sensitive" who detects nearby magical talent, reveals that if the Emrys family dies out, the alternate timeline collapses.
During a lesson, Jax's inquisition leads him to an encyclopedia entry for King Arthur. Melinda confirms that Riley descends from Arthur and that "Pendare" is an alteration of "Pendragon." She reveals that the Dulac clan leader, Ursula Dulac, arranged the assassination of Riley's entire family at an engagement party for his sister, Alanna. Riley, then about thirteen, was the sole survivor. He has hidden ever since, as claiming his seat at the Table, a council of Transitioner lords, would expose him to the Dulacs.
Defying Riley's rules, Jax befriends Evangeline. Over their first conversation, she reveals she has not spoken to anyone in five years. Riley explains that a Kin lord named Wylit once attempted to alter the spell, causing magical backlash that devastated a Midwestern town. Jax's father was hired by Wylit to find Evangeline but instead traded intelligence to Riley, including planting a spy among Wylit's vassals, in exchange for Jax's protection. Rayne's betrayal of Wylit led to his murder.
Searching for information about the eighth day, Jax joins an online forum using the same password as his email, a mistake that leads to his kidnapping by Terrance, a Normal who exploits Transitioners for robberies. The Donovan family, Transitioner classmates who track by scent, rescues Jax, but he conceals their help from Riley, with catastrophic results.
When Miller Owens, Riley's spy among Wylit's vassals, warns that Wylit's forces are heading for the Taliesins, Riley departs with his vassals and Deidre Morgan, whose powerful Transitioner clan has long expected her and Riley to marry. In Riley's absence, Wylit's forces attack. The Donovans, who detected a powerful scent on Jax during their visit, have sold Evangeline's location to Wylit's vassal John Balin. Evangeline swears Jax in as her vassal, an oath forcing Balin to honor her right to protect him. Balin's men seal Evangeline in a casket for transport through the Normal timeline and take Jax captive.
At a farmhouse in Ohio, Jax meets Miller Owens. Miller confirms his loyalty via texts on his own phone and outlines three options: kill Wylit, rescue Evangeline, or kill Evangeline before Wylit can use her. Jax is taken to Mexico, where Evangeline is brought before Lord Myrddin Wylit, a Kin lord horribly disfigured by his previous failed spell. Wylit reveals his true plan: not to free the Kin but to push the seven-day timeline out of existence, eliminating all Normal humans. He has assembled the components: Evangeline, the mummified body of Niviane, and Excalibur, Arthur's honor blade, a dagger used to channel magic. The ceremony takes place at Teotihuacán, an ancient city in Mexico whose original civilization was destroyed by a similar spell.
Atop the Pyramid of the Sun, Wylit slashes Evangeline's arms with Excalibur and announces he will sacrifice Riley, whom Miller has delivered in an apparent betrayal. As cracks of light split the sky, Jax releases a light spell Evangeline planted in his mind, creating a blinding flare that incapacitates Wylit. Miller signals reinforcements: His betrayal was a ruse. A Morgan helicopter attacks while the Crandalls and Michael Donovan emerge from tunnels beneath the pyramid. Miller kills Balin but is mortally wounded. Jax cuts Evangeline free, and she shoves Wylit off the summit with Riley's dagger, impaling and killing him.
Despite Wylit's death, the spell remains unstable. Evangeline and Riley clasp hands at the altar, she holding his dagger and he holding Excalibur. She affirms the spell's original purpose, invoking the sacrifices of Miller, her brother Elliot, and Jax's father. The allies link hands, but the cracks persist. Riley kisses Evangeline, channeling powerful emotion into the spell. The sky heals, and at midnight, Evangeline vanishes into Thursday, confirming the eighth day is intact.
Miller dies from his wounds. Riley reveals Miller had been engaged to Alanna before the family massacre. Deidre releases Riley from the expected engagement, recognizing his feelings for Evangeline. Riley relocates everyone to a Pennsylvania mountain cabin and plans to come out of hiding, claim his seat at the Table, and find Evangeline's sister Adelina before the Dulacs do. Mrs. Crandall, Arnold's wife, urges Riley to send Jax back to Naomi as Rayne wished. Riley agrees that Jax needs school and serves mainly as a comfort to Evangeline while she adjusts. When Evangeline offers to release Jax from his oath, he declines. He no longer wants a Normal life and refuses to leave.