In a world parallel to the ordinary one, major holidays are governed by royal families who wield magic fueled by the joy their celebrations generate. Christmas, the most powerful Holiday, is ruled from Claus Palace in Greenland by King Claus, the reigning Santa. His eldest son and heir, Prince Nicholas "Coal" Claus, is a charming prankster whose reputation in the Holiday tabloids is one of reckless irresponsibility.
The story opens during summer break after Coal's sophomore year at Yale. Coal accessed the North Pole's database of unfulfilled Christmas wish letters and magically granted every outstanding wish for children in New Koah, a small Pacific Islands nation. Driven by heartbreaking letters from children in need, he believed he was doing something meaningful. Instead, the flood of millions of gifts and cash triggered hyperinflation, riots, and robberies. Drunk at a bar after ruining his girlfriend Lily Lentora's birthday party, Coal takes a call from his father, who frames the disaster as a PR problem. Coal lashes out, accusing his father of never using Christmas's resources to help anyone and blurting out that Christmas's joy was not enough to keep their mother from leaving when Coal was eight.
Coal flees to the bar's back alley, where he encounters a stranger with glossy black hair and intense dark eyes. Coal pours out his anguish, and the stranger offers a perspective that lodges in Coal's mind: Happiness is not one grand gesture but a foundation built from many small things. A kiss follows before the stranger vanishes. Coal's friend Iris Lentora, the Easter Princess and Lily's younger sister, finds him alone.
A year and a half later, during Christmas break of Coal's senior year, Coal returns home with his younger brother, Kris. At the annual tree-trimming event, Coal's father blindsides him: He has secretly enrolled Coal in a master's program without his consent and publicly declared that Coal has begun courting Iris, with an engagement expected by season's end. Neither Coal nor Iris wants this marriage, but Iris explains she cannot refuse because factions within Easter's court challenge her family's legitimacy, since her father, King Neo, married someone outside the Holiday world. Coal agrees to go along for her sake. Kris, who has quietly loved Iris for years, says nothing. Coal's father later reveals the wedding will take place at the Christmas Eve Ball, less than a month away.
Envoys from Halloween arrive to object to the union as a power grab. Coal's father publicly offers to let Halloween compete for Iris's hand but privately reveals the competition is a ruse.
At the welcome party, Coal recognizes Halloween's prince, Hex Hallow, instantly as the stranger from the alley. When Coal approaches, Hex reveals he knew who Coal was all along. Then he corrects a belief Coal has carried for over a year: "You weren't the one who initiated it" (86). Hex kissed Coal, not the other way around.
Over the following days, Coal and Hex circle each other through the fake competition. In a private moment, Hex reveals he knows the competition is a sham. He is effectively a prisoner: Christmas threatened Halloween, and Hex came willingly to protect his Holiday. Coal is horrified and promises to fix things. Hex insists he does not need saving, then kisses him.
Coal confronts his father and discovers the full scope of Christmas's corruption. For years, his father has coerced over a dozen winter Holidays, including Yule, Thanksgiving, and New Year, into tithing large percentages of their joy to Christmas, using damaging information as leverage. Christmas derives more than three-quarters of its claimed joy from these forced tithes. The revelation destroys Coal's remaining hope that his father was the idealistic leader he once seemed.
Coal and Hex's relationship deepens. They discuss Halloween's collaborative model with its autumn allies, where joy is pooled equally, and Coal proposes creating a similar winter Holidays collective. He and Kris draft letters inviting the coerced Holidays to the Christmas Eve Ball. Coal begins training in Christmas's operations and transforms court events, replacing stiff formality with genuine celebration. He confides his shame over the New Koah disaster to Hex, who asks the one question no one ever has: why Coal did it. Hex tells him the mistake was misguided, not irresponsible, and that what matters is how he proceeds.
A raw conversation with Kris reveals how deeply their mother's abandonment shaped them both. Kris confesses his deepest fear: that Coal will eventually leave, just as their mother did. Coal promises he never will.
Then Coal's father discovers the response letters. In a furious confrontation, he accuses Coal of betrayal. When Kris shouts at their father to stop, Coal sees his father's arm move and throws himself between them, begging. His father was only gesturing, not striking, but the reaction reveals how deeply Coal fears him. His father threatens to expose Halloween's prior negotiations with Christmas to the autumn collective, destroying Halloween's alliances. Coal surrenders to protect Hex. He rushes to Hex, who reveals that his older sister Raven originally wanted to ally with Christmas and died in a car accident days after those negotiations ended. Hex has complied with Christmas's threats to protect Raven's legacy. He urges Coal to proceed with the collective anyway, but Coal refuses, saying he will not let his father hurt the man he loves. Hex argues they were always destined to choose duty over each other. They fight bitterly, and Hex is sent home.
Devastated, Coal finds support in Kris, who tells him, "You are not a burden" (299). Then Coal realizes his father's threats are a bluff: Releasing damaging information would require revealing Christmas as the source, destroying the wholesome image his father built. Coal decides to proceed. Wren, his father's head assistant, reveals herself as an ally, fabricating evidence that rescission letters were already sent to buy Coal time.
At the Christmas Eve Ball, Coal stands his ground. He tells his father the tithes end now and calls the bluff. Kris brings the winter Holiday representatives into the room, and Coal addresses them, acknowledging Christmas's theft and proposing the collective. His father says only, "I would not have hit you or your brother" (321), acknowledging Coal's fear.
Coal and Iris cancel the wedding. Iris reveals that Hex returned that morning. He stands on the dance floor in a cherry-red suit, a union of Halloween and Christmas colors. Hex confesses he began falling in love in the bar alley: He had gone there on the first anniversary of Raven's death, wanting to confirm the Christmas Prince was as bad as his father, but found the opposite. They kiss publicly, with cameras flashing. Coal declares his love, and Hex says it back.
On Christmas morning, the four friends gather as meetings with the winter Holiday representatives begin. Coal and Hex plan a diplomatic visit to Halloween, the first step in building a life together across their Holidays. The novel closes with Coal shouting down the palace hall, "My boyfriend is insanely sexy!" and imagining the tabloid headline: Prince Coal of Christmas is inexhaustibly in love with Prince Hex of Halloween.