Sadie is an ambitious detective with the Metropolitan Police Service in London. She is currently on leave in Cornwall after leaking information about a missing persons case that she believes the department mishandled. Driven by her own past as a teen mother who gave up her daughter, she struggles to let go of cases involving fractured families. While staying with her grandfather, she discovers the abandoned Loeanneth estate and becomes fascinated by a 1933 cold case.

Alice is the second-oldest daughter of the Edevane family, an imaginative teenager in the 1930s who dreams of writing detective novels. In 2003, she is a highly successful author in her eighties living in London. She harbors deep guilt regarding the night her baby brother vanished, believing her own actions indirectly caused the tragedy. She closely guards her privacy until Sadie begins asking questions about her family's past.

Eleanor is the matriarch of the Edevane family, carrying the heavy burden of managing her husband's trauma following World War I. Originally a free-spirited young woman who loved adventure, she adopts a strict, authoritative demeanor to keep her household functioning. She fiercely protects her children while trying to maintain the appearance of a perfect family life despite her growing private struggles.

Anthony is the youngest son of a wealthy aristocratic family who inherits the entire fortune after his relatives perish on the Titanic. Originally studying to be a surgeon, his experiences in the trenches of World War I leave him suffering from severe shell shock. He struggles to control his emotions and unpredictable reactions, relying heavily on Eleanor to maintain order in their home.

Benjamin is a twenty-six-year-old itinerant worker employed to help the head gardener at the Edevane estate in 1933. Raised by archaeologist parents, he prefers a simple, nomadic lifestyle rather than settling down. He serves as an attentive listener for Alice's story ideas while bringing a sense of grounded calm to the tense atmosphere surrounding the family.

Theo is the youngest child of the Edevane family. His sudden disappearance from his crib during the Midsummer Eve's party fractures the family irreparably, leading them to abandon the Cornwall estate. His vanishing leaves a mystery that remains unsolved for seventy years.

Bertie is Sadie's gentle, supportive grandfather who lives a quiet life in the Cornwall countryside. He previously took Sadie in when she was a pregnant teenager, offering her stability when her parents turned her away. He enjoys peaceful routines, walking his dogs Ash and Ramsey, and offers Sadie a refuge from her stressful career.

Clementine, known as Clemmie, is the energetic and tomboyish third daughter of the Edevane family. She prefers climbing trees and flying model planes over traditional ladylike pursuits. She dotes on her baby brother Theo and is hit particularly hard by his disappearance, changing from a boisterous girl to a quieter, more withdrawn adolescent.

Deborah is the eldest Edevane daughter, who prefers the bustling life of London to the isolation of the Cornwall estate. She is practical and grounded, often acting as a stabilizing force among her siblings. In her later years, she maintains a strict routine of meeting Alice annually to remember their mother.

Constance is Eleanor's mother, a bitter and cantankerous woman who lost her ancestral wealth due to her late husband's poor investments. She lives with the Edevane family at Loeanneth but shows little gratitude. She uses her advancing age and supposed senility as cover to quietly observe and judge the household staff and guests.

Daffyd, often called Mr. Llewellyn, is a former physician and longtime friend of the Edevane family. He is the author of a beloved children's book inspired by Loeanneth's grounds. He acts as a confidant for Eleanor, understanding the medical reality of Anthony's condition, but his presence at the estate during the Midsummer party causes tension.

Peter is Alice Edevane's personal assistant in 2003, originally hired temporarily after impressing her with his literary knowledge. He is a diligent bookworm who manages Alice's correspondence, meals, and research. His curiosity is piqued when he notices Alice acting secretively regarding Sadie Sparrow's letters.

Donald is Sadie's partner at the Metropolitan Police Service. He is pragmatic and nearing retirement, preferring to follow protocol rather than make waves. While he refuses to report Sadie for leaking case details to the press, he insists she leave London until the resulting departmental anger cools down.

Clive is a retired police officer who was part of the original investigation into Theo Edevane's disappearance. He has kept the old case files, unable to let go of the unsolved mystery. He proves to be an invaluable resource for Sadie, offering institutional knowledge and access to Loeanneth.

Nancy is the grieving but resolute mother of Maggie Bailey, the missing woman whose case Sadie is investigating. Refusing to accept the police's conclusion that Maggie walked away from her life, Nancy persistently contacts Sadie, providing new details and hoping the detective will keep searching for the truth.

Maggie is a young mother who vanished, leaving behind a note that the Metropolitan Police Service believes is proof of abandonment. She fought a difficult custody battle for her daughter, leading her own mother to firmly believe she would never have left voluntarily.

Howard is a young man who serves alongside Anthony in World War I. He falls deeply in love with a French woman during the conflict and attempts to desert to start a new life with her. His actions, and the tragic consequences that follow, become the start of Anthony's severe trauma.

Nanny Rose is the former caretaker for the Edevane children who is let go shortly before Theo's disappearance. She is deeply attuned to the family dynamics and provides careful, watchful care for the children, making her sudden dismissal a point of interest in the police investigation.

Nanny Bruen is the caretaker hired by the Edevanes after Nanny Rose's dismissal. She is generally favored by Constance for her strict adherence to traditional roles, but she proves to be an unreliable guardian on the night of the Midsummer Eve's party.

Flo is a lifelong friend of Benjamin Munro who lives in London and runs a shop. She provides a sense of stability and connection for Benjamin, who is otherwise an itinerant worker.