Annie is an aspiring writer living at her inherited country estate in Castle Knoll. She manages her mother's practical affairs while using her great-aunt's historical archives to investigate cold cases. Her sharp intellect and methodical approach drive her to travel to London when a bloody animal heart appears on her mother's doorstep, leading her to uncover a fresh murder.

Frances is an intelligent and determined psychology student establishing herself in the academic and social circles of 1968 London. She actively studies the homicidal mind while collecting secrets as a defensive measure against those who might harm her. Her meticulous diary entries document her dangerous interactions with the wealthy Torrence family, leaving a legacy of clues for her great-niece.

Vera is a wealthy socialite married to an older, controlling heart surgeon who frequently destroys her artwork. Feeling trapped and fearing for her life, she collects blackmail material against the powerful men in her orbit to gain leverage. She confides in Frances about her desperate situation shortly before the police announce her gruesome murder.

Crane is a steadfast police detective working in London who shares a growing bond with Annie. He balances his official duties with an open-minded approach, often utilizing Annie's historical files to aid his cases while managing his past relationship with the victim, Fliss. His professional demeanor masks underlying guilt regarding his ignored messages from his ex-girlfriend shortly before her death.

Max is an arrogant and entitled university student in 1968 who uses his family's wealth to intimidate others. He takes pleasure in humiliating women, demonstrated by his cruel commissioning of a nude painting of his ex-girlfriend. In the present timeline, he works at the front desk of a Knightsbridge art gallery, maintaining his hostile attitude.

Laura is a celebrated modern artist and Annie's mother. She is eccentric and often relies on her daughter to manage her daily life. Her successful career was launched in the 1990s following the sale of her early collection, though she harbors deep anxiety about her past and the sudden reappearance of her ex-husband.

Sam is Annie's estranged father and a known conman who abruptly re-enters Laura's life. He offers conflicting stories about the origins of Laura's artistic fortune and his reasons for disappearing. His sudden presence creates friction within the family as he attempts to deflect suspicion onto others.

Brian is a former police officer whose early career intersected with the Torrence family and Dr. Huntington in the 1960s. In the present, he contacts Detective Crane under false pretenses to share explosive information about the historical case. He is deeply motivated by a personal tragedy involving his sister's medical treatment.

Elaine is a university student in 1968 whose cyclical relationship with Max Torrence subjects her to his manipulative games. She exhibits a mix of insecurity and social ambition, often acting condescendingly toward Frances despite their friendship. Decades later, she approaches Annie claiming to have information about the historical events.

Fliss is a young woman from Castle Knoll who recently secured a position as Laura's artistic apprentice in London. She repeatedly attempts to contact her ex-boyfriend, Detective Crane, about a terrible mistake before falling victim to a gruesome murder.

Dr. Huntington is a prominent heart surgeon and Vera's much older husband in 1968. He uses his professional authority and status to control his wife, dismissing her ambitions and pathologizing her behavior to others.

Ford is a wealthy nobleman and Frances's complicated former flame. He provides her with strategic support and physical safety as she confronts the dangerous conflicts involving the Torrence family in London.

Peony is a late fortune-teller from Castle Knoll. She issued ominous prophecies about a beating heart that continue to influence Annie's decisions and frame her understanding of the unfolding mystery.

Professor Dane is a supportive faculty member at University College London. He recognizes Frances's intellect and actively encourages her studies by enrolling her in his advanced lecture on the homicidal mind.

Susan is Brian's sister, an unhoused woman whose tragic death on an operating table in the 1960s becomes a crucial piece of evidence connecting the past to the present.

Kabir is a pathologist and a professional contact of Detective Crane. He provides medical expertise by examining cold case files, uncovering overlooked details about historical causes of death.

Archie is a friendly resident of Castle Knoll. He provides Annie with crucial background information about Fliss's past relationship with Detective Crane.

Beth is Archie's granddaughter and a friend to Annie. She assists with the daunting task of searching through Frances's extensive historical files for specific contact information.