A professional assassin fails a mission and dies, only to open her eyes in the body of a newborn baby. She has been reincarnated as Serena, the daughter of a duke in a world of swords and magic. Retaining all the memories and skills from her previous life as a cold-blooded killer, she believes that pretending to be a noble will be an easy act to maintain.

Serena quickly discovers that navigating aristocratic society is a different kind of challenge than any assassination mission. Her mother, Amaryllis, is a flighty and naive optimist who cannot understand her daughter's chilling demeanor. To make matters worse, her adopted sister, Hera, is a scheming girl who deceives everyone around her in an attempt to push Serena out of the family. Serena tries to adapt, using her icy glare and fierce counters to fend off harassment at tea parties and manage the complex social life at the academy. However, her inability to understand or express normal emotions leaves her isolated and ostracized by her peers.

The conflict escalates when Serena learns that Hera has become engaged to the second prince. Seeing this as a final betrayal and a dead end, she decides to run away from home. Her escape is interrupted when she literally bumps into the first prince, Evan. His unexpected interest in her sets Serena on a new path, forcing her to navigate political intrigue and romantic entanglements for which her assassin training never prepared her. As she charges forward through high society, this former killer who has never known love finds herself confronting emotions and relationships that challenge everything she thought she knew about survival.