Evangeline Fox, a young woman from the South, is desperate to escape a tragic fate. Faced with an impending arranged marriage to a man she doesn't love, she makes a daring deal with Jacks, the mysterious Prince of Hearts from the North. In exchange for saving her from this doomed future, she agrees to serve Jacks for seven years.

Upon entering the enchanting and magical North, Evangeline quickly realizes the complexities of the world she's entered. Jacks introduces her to the Fates, a group of supernatural beings who control different aspects of destiny. Each Fate represents a unique force, from love to rejection, and Evangeline finds herself entangled in their affairs.

As the story unfolds, Evangeline discovers that she is the key to a prophecy involving the Valory Arch, a magical doorway with the potential to alter fate. The prophecy dictates that only a key born in the South, crowned in rose gold, can open the arch. Evangeline, with her distinctive rose gold hair, becomes the center of attention and speculation.

Amidst the intrigue, Evangeline wakes up in Jacks' arms, feeling embarrassed but convinced that he is indifferent to her. She learns that the woman who saved her life is LaLa, another Fate representing rejection and unhappily ever afters. Jacks, in his callous manner, reveals that LaLa is the one who poisoned Evangeline with her tears, unknowingly sold to an unknown buyer.

The trio embarks on a journey to Chaos, another Fate with a reputation for being a vampire. The revelation of Chaos being a vampire adds a layer of danger and tension to their quest. Along the way, Evangeline forges a bond with LaLa, discovering the complexities of friendship in this magical world.

As they seek answers about the murder of Prince Apollo, Jacks and Evangeline encounter Chaos, who provides crucial information about a stranger's request for a rare magical poison. This revelation fuels their determination to uncover the truth, leading them to a confrontation with transformed vampires and a surprising encounter with Luc, who is revealed to be alive but now a vampire.

The narrative takes a dark turn when Jacks, infected with vampire venom, undergoes a painful transformation. Evangeline, torn between fear and concern, manages to help Jacks survive the night without turning into a vampire, revealing her growing emotional entanglement with him.

The plot intensifies as Evangeline discovers the latest gossip, painting her as a murderer with Tiberius set to become king. Despite warnings, she decides to confront Tiberius with the truth serum, only to unravel a shocking revelation – Tiberius himself killed Prince Apollo, believing Evangeline to be the key to stopping the Valory Arch from opening.

In a dramatic turn of events, Tiberius admits to being part of the Protectorate, a secret order aimed at preventing the arch from opening. This revelation puts Evangeline's life in jeopardy, but using the truth serum strategically, she turns the tables on Tiberius, exposing his guilt and saving herself.

The narrative reaches a climax as Evangeline, betrayed by Jacks' manipulations involving Prince Apollo, grapples with her feelings of anger and deception. A new revelation about Apollo being in a suspended state further complicates her journey, leaving her emotionally shattered.

The story concludes with Evangeline contemplating her future, questioning her beliefs in love and destiny. The intricate web of magical elements, prophecies, and complex characters in Once Upon a Broken Heart creates a rich and captivating narrative that explores the blurred lines between fairytales and the harsh realities of destiny.