On the fiftieth day of the Centennial, the Betwixt Ball begins as both celebration and threat: at midnight, the rule against killing will expire. Isla dresses in a revealing Wildling gown made defensive with hidden weapons and chain mail, knowing the protections may not save her if another ruler attacks. She enters the ballroom among the other rulers, sees Celeste across the room, and notices Cleo watching her after the failed assassination attempt.

Isla dances with nobles to appear unafraid while observing the rulers and the political tension around her. Azul acknowledges her from afar, prompting possible rumors of alliance, while Oro remains surrounded by Sunlings with his worsening condition hidden beneath his sleeves. Overwhelmed by gossip, stares, and the approach of midnight, Isla leaves the ballroom and steps onto an interior balcony.

Grim appears there and dances with Isla, then hides them both from approaching nobles by making them invisible. Isla asks to leave the ball for a short time, and Grim takes her through the castle to an underground chamber beside dark water. Their charged intimacy is interrupted when Grim gives Isla an invisible black-diamond necklace that will summon him if she touches it, promising he will come for her. When they return, Grim leaves suddenly and oddly suggests Isla dance with Oro.

Celeste finds Isla and asks whether they should fight or hide when midnight comes. Isla chooses to fight, deciding she will not survive by running. Before Celeste can respond, the castle violently collapses: chandeliers fall, the floor splits, guests die, and rulers use their powers to shield or heal whoever they can. Isla realizes this is not an attack but Lightlark itself falling apart because the curses have gone unbroken for too long.

Seeing Oro incapacitated on the floor, Isla runs through the destruction to reach him because only his power can stop the collapse. Oro tells Isla to leave, but Isla refuses and violently urges him to act before his brother’s sacrifice and everyone’s losses become meaningless. Oro finally drives his hands into the marble and releases enough power to still the room, then collapses. As Celeste helps Isla leave, Isla understands that the Centennial may truly be Lightlark’s last chance, and the island may not survive the remaining days.