After the dinner, Harlow sneaks out of the manor. She reflects on her family’s power, which began when her ancestor, Alistair Carren, was blessed by Harvain, the Divine (God) of Fortune with the ability to see magic and wield holy fire against the Drained. Harlow’s own magic—a poison kiss—manifested accidentally when she was six, killing a playmate. Her parents embraced the deadly gift as a tool for the family.

Now, Harlow lives a double life as the “Poison Vixen,” a vigilante who kills abusive men. She would kill Rafe, Aidia’s abusive husband, in a split second, but is under strict instructions by her family to hold back. Presently, Harlow uses a glamour pendant from Aidia (who has concealing magic) to alter her appearance before checking a secret mailbox for requests. At a pub called Guardian’s Crossing, she meets her accomplice, Beatrice “Bea” Grange, who has a new client needing a rush job. This is against their rules, but Harlow agrees to take the case after telling Bea about her betrothal. The bruised client identifies her husband, Ron, and says he can be found at the Heartless Haven Pub.

At the Heartless Haven Pub, a city-wide power outage causes Harlow’s glamour to fail, forcing her to proceed with the mission using her own face. She identifies her target, a handsome man who calls himself Asher. Harlow introduces herself as “Stellaria,” reflecting upon the myth of the Divines Stellaria and Asher, which every child in their world knows. When Stellaria, the Divine of Stars, learned that her father had kidnapped her husband Asher, Divine of Endings, she covered the world in darkness for four days. Pom, Stellaria’s father and the Divine of Malice, cursed the ground on which Asher was imprisoned, where no one of life or death could enter. Stellaria created a man who was neither living nor alive to fetch Asher. The Divine couple were happily reunited, but the undead man became first of the Drained, the cursed life form that haunts their world.

Turning her mind to her mark, Harlow flirts with him, as part of her usual modus operandi. Asher is immediately charmed by her and takes her to a private room upstairs.

Harlow finds herself uncharacteristically attracted to Asher. Despite this, she summons her poison and kisses him passionately and repeatedly, expecting him to die within moments. To her astonishment, he is completely immune to her magic. Confused and unnerved by her power’s failure, she abruptly ends the encounter. Claiming she is late for another engagement, she flees the room, leaving her target alive and unharmed.

The next day, as Harlow prepares to meet her fiancé’s family with her maid, Marie, she solidifies a plan. She will use the arranged marriage to earn a favor from her father: the key to a secret tunnel out of the city, which she intends to use to escape with Aidia. Her mother, Liza, confirms the marriage is a spy mission. They do not trust the Havenwoods and want Harlow to investigate their connection to a rebel leader named Rochelli. Harlow agrees to the task in exchange for her freedom and the tunnel key once the job is complete, a deal her father accepts.

Her bodyguard, Gaven, escorts her to the meeting. When she asks him about her betrothed, Gaven gives a vague but reassuring description, noting he is handsome and much younger than her last husband. Harlow steels herself before the sitting room door, preparing to meet the man she will have to deceive.

Harlow enters the sitting room and meets her future in-laws, Philip and Evangeline Havenwood. She is surprised to find she cannot see their magical auras, which is highly unusual. Their son and her betrothed, Henry, is running late.

When Henry finally arrives, Harlow is shocked to realize he is “Asher,” the man she failed to kill the previous night. He recognizes her as “Stellaria” but plays along, hiding their prior meeting from their parents. After all parties sign the marriage contract in blood, Harlow’s father announces her deadly magical power: Harlow possesses a poison kiss.

The perspective shifts to Henry, who is stunned to realize his fiancée tried to murder him. He decides not to expose Harlow, deciding to discover on his own who hired her and to get his revenge. At the Havenwoods’ request, the Carrenwells provide a demonstration of Harlow’s power, and she kills a paid volunteer with a kiss. In turn, Henry’s mother reveals that Henry’s magic is healing. To prove it, Harlow’s father cuts her arm, and Henry heals the wound with his touch, feeling her pain as he does so. Henry reflects that the Carrenwells are callous toward Harlow, but that this is not surprising given that they just casually witnessed a man die for no reason.

Later, Henry insists on speaking with Harlow alone in the gardens. He confronts her about the murder attempt and reveals he wore a ring that hid his magical aura. She admits she was hired to kill him. When he tests her physically, she easily overpowers him. Before she can leave, he stops her by asking what she knows about the rebel leader Rochelli. Harlow says she knows nothing and abandons Henry in the center of the hedge maze.

The stressful confrontation with Henry triggers one of Harlow’s debilitating, migraine-like episodes. Her bodyguard, Gaven, helps her back to her room, where the intense pain causes fragmented memories of her father’s abuse to surface. Harlow remembers that Aidia, the rebellious one, bore the most brunt of his cruelty. After Harlow recovers, Aidia visits, even though slipping out without Rafe’s permission will endanger her. Harlow’s parents know Rafe abuses her, yet they ignore the situation because of Rafe’s status in Lunameade.

To restore Harlow’s strength, she and Aidia go to the secret Carrenwell Family Well in the manor’s cellar. A well is the source of magical power in the world of the novel. Harlow tells Aidia that someone hired her to kill Henry, and they deduce it was a setup, likely orchestrated by the rebel leader Rochelli. Aidia teases Harlow about her clear attraction to her new fiancé. Harlow then reveals her plan: She will complete her mission in Mountain Haven, earn the key to the secret tunnel from their father, and use it to escape the city with Aidia forever. To Harlow’s dismay, a defeated Aidia insists that escape is impossible.

The perspective shifts to Henry, who meets with a rebel contact in Lunameade before debriefing with his father. He mentions Harlow’s combat skill but withholds that she tried to kill him. Later, Henry joins his friends and fellow huntsmen, Carter and Bryce, to scout the city. Bryce reports on the city watch patrol patterns and confirms that Rochelli is a mysterious but popular figure.

Their reconnaissance is interrupted when they witness city guards violently forcing an old man to give blood for the city’s magical Blood Well. Bryce informs Henry that the Carrenwells recently doubled the frequency of this mandatory blood tithe, deepening the citizens’ resentment. The injustice fuels Henry’s desire for vengeance against the Carrenwells for their role in the attack that destroyed his home and killed his sister 10 years earlier.

At a formal feast, Harlow speaks with her brother Kellan, who is captain of the city watch. Out of her eight siblings, Kellan is Harlow’s favorite brother. Kellan asks Harlow about Henry and discusses his ongoing hunt for the Poison Vixen—unaware that he is speaking to the vigilante herself. When Henry arrives, he and Harlow continue their charged exchange of threats. To test Henry’s immunity to poison magic again, Harlow secretly adds a different venom to his wine, which he drinks without any effect.

Aidia’s husband, Rafe, asks Harlow to dance. He taunts her with crude innuendoes and reveals that he knows she met Henry before the official introduction and that her kiss is poison. Harlow realizes with horror that Rafe must have manipulated or forced Aidia to reveal her secrets. Rafe possesses manipulation magic, which he often uses against his enemies. Rafe is lecherous toward Harlow, whom he had originally hoped to marry. However, the Caldwells arranged his match to Aidia. Henry approaches Rafe and Harlow; Rafe leaves before Henry can intervene. Henry pauses in front of Harlow, senses danger, and tackles Harlow to the ground an instant before a bomb detonates outside, shattering the ballroom windows.

Henry throws himself against Harlow to protect her from the falling glass. It is clear rebels have orchestrated the attack. Gaven is suspicious of Henry’s unnaturally quick reaction. Henry heals a wound on Harlow’s arm, and they learn Kellan has been injured. Using one of the manor’s secret passages, they find Kellan after he fought off several rebels. Henry heals Kellan’s stab wound while Harlow and Gaven fight more attackers. Kellan then uses his powerful compulsion magic to force the remaining rebels to surrender.

The three find Henry’s parents in the library, where his mother, Evangeline, has been wounded. Kellan uses his own magic to take her pain while Henry heals the injury. Fearing for their safety, Henry’s father declares they will leave for Mountain Haven in two days. The abrupt change makes Harlow suspicious. Henry invites her for a drink on her last night in Lunameade, which she accepts, calling it a “hostage negotiation.” Harlow is to travel with Henry’s family and wed him a few days later. Gaven is the only person from home who will accompany her.

On her last night in Lunameade, Harlow goes to Guardian’s Crossing to take on one last job as the Poison Vixen. She tells Bea she suspects Rafe set her up to kill Henry. Bea reveals that she and her partner, Josie, plan to continue the Vixen’s work while Harlow is away to avoid suspicion from Kellan. Harlow objects, but Bea insists.

Harlow proceeds to another bar, Ashen Eclipse, for her final target. Henry appears unexpectedly and ruins her plan. He reveals he is colorblind—a result of the attack on his home—and thus cannot see her glamour magic (Harlow changes her appearance for her assignments). He also shares that his older sister, Holly, was the true heir of their family and died in the attack of the Drained, 10 years ago. He then blackmails Harlow into spending the evening with him and his friends on pain of revealing her alter ego to Kellan, forcing her to abandon her mission.