The story begins at the 17th Royal Academy's graduation party, a seemingly ordinary event that is abruptly shattered by Prince Jean's public declaration to break off his engagement with Lady Alessandra. Alessandra, the daughter of the influential Marquis of Cremont, is shocked, as is the entire assembly. While the other students are caught between sympathy and morbid curiosity, Alessandra, despite her outward composure, seethes with anger, attributing the prince's actions to the manipulative influence of Sophie, a student of lesser standing who has been charming her way through the academy. Sophie, dressed in white with braided purple hair, feigns distress and leans on the prince, who readily supports her.
Alessandra realizes her oversight in underestimating Sophie's political maneuvering within the academy's supposed equality. As she attempts to leave to consult her father, Prince Jean's entourage blocks her path, and the exit is locked. Alessandra understands Sophie's strategy: to solidify the engagement annulment before Alessandra can leverage her family's influence outside the academy's confines. A tense standoff ensues, culminating in a male student collapsing, his face alarmingly pale. To Alessandra's horror, he then violently attacks and kills the son of the High Priest, initiating what would become known as the "Night of the Dead Festival" or "Zombie Party Night."
Chaos erupts as the students panic. The prince orders his guards to apprehend the attacker, who is quickly subdued. However, the situation escalates when the subdued student bites another guard, who then turns on the prince, fatally wounding him. Sophie flees in terror, while the remaining guard and the newly turned prince become zombies, attacking the remaining students. The initial chaos is amplified tenfold, turning into a desperate fight for survival.
Alessandra finds herself isolated amidst the pandemonium. She observes the fallen prince and realizes her engagement is now physically broken. She notices a ceremonial saber and, despite its unsuitability for combat, picks it up. She discovers that decapitation is an effective way to stop the undead. Meanwhile, Sophie, though terrified, uses her agility and unique fighting style, incorporating high heels and kicks, to defend herself.
Alessandra, recognizing the need for a strategic retreat, identifies the stage as a defensible position due to its height and limited access points. She dispatches the prince's zombie corpse and, surprisingly, forms a temporary alliance with Sophie, who had been hiding. They agree to work together to survive, acknowledging their shared predicament despite their vastly different personalities and backgrounds.
Their alliance is tested as they fight off waves of zombies. Alessandra's superior swordsmanship, honed by her grandfather's teachings, proves effective, while Sophie's unconventional kicking techniques are surprisingly potent. They realize their weapons and footwear are reaching their limits. Alessandra proposes a daring plan: to break through the locked exit.
They fight their way to the main door, a heavily reinforced structure. Alessandra, using a powerful technique called "Great Serpent's Tail," clears a path, but her ceremonial sword breaks. She then acquires a more robust sword from a fallen guard. Sophie, despite her exhaustion, continues to defend Alessandra as she attempts to cut through the door's locking mechanism.
Alessandra, with immense concentration and a newfound determination, manages to sever the lock. They escape through the door and find themselves in a deserted corridor. Sophie, ever the schemer, attempts to shift blame for the prince's death onto Alessandra by feigning pursuit by the "madwoman" who killed the prince. However, Alessandra swiftly decapitates a pursuing guard, revealing that the undead are now within the palace itself.
Realizing the gravity of the situation, they decide to use a secret passage Sophie knows about, leading them through the palace. They navigate the dark corridors, their contrasting personalities clashing but their shared will to survive uniting them. They eventually reach a junction with two paths: one leading outside the castle and the other to the prince's chambers. Alessandra, embracing the weight of responsibility, chooses the path towards the royal chambers, believing it to be the most strategic move for the kingdom's survival.