Sadie is an introverted physician and mother of two boys who recently relocated her family from Chicago to an isolated island in Maine. She is dealing with the aftermath of professional and personal troubles in Chicago and harbors a deep sense of unease about their new home. When a neighbor is murdered, Sadie's growing paranoia and memory lapses complicate her desperate attempt to investigate the crime and protect her family.

Will is Sadie’s charismatic, attractive husband who works as a college professor. He moves his family to Maine in pursuit of a fresh start after his sister's death and his own infidelity in Chicago. Often acting as the supposed voice of reason, Will presents a calm, capable demeanor that contrasts sharply with Sadie’s escalating anxiety.

Camille is an edgy, fiercely independent woman from Sadie and Will's past in Chicago. She possesses a coarse, transactional personality and an intense jealousy regarding anything she considers hers. Driven by a volatile mix of desire and protectiveness, she inserts herself into Will and Sadie's dynamic with calculated determination.

Mouse is an imaginative, deeply empathetic six-year-old girl who loves spending time and playing games with her father. Her once-happy domestic life begins to crumble with the arrival of her new stepmother, whom she dubs Fake Mom. She tries to survive by becoming small and unnoticed, turning to her imagination to cope with the escalating cruelty in her home.

Imogen is Will’s sixteen-year-old niece who recently lost her mother to suicide, a trauma made worse by the fact that she discovered the body. Now in her uncle and aunt's custody, she copes with her grief through a dark, gothic aesthetic and rebellious, antisocial behavior. Her hostility masks a deep psychological burden and complex secrets regarding her family's past.

Otto is Sadie and Will’s introverted, artistic fourteen-year-old son. He carries the emotional scars of being brutally bullied at his previous school in Chicago, an ordeal that culminated in him bringing a knife to class. He is sensitive and vulnerable, sharing his mother's unease in their new environment.

Tate is the ten-year-old son of Sadie and Will, characterized by a sunny, carefree outlook that heavily contrasts with his older brother's gloom. He adapts quickly to his new home and shares a close bond with his father, though his relationship with his mother becomes strained by her unexplained memory gaps.

Officer Berg is the local policeman leading the investigation into the brutal murder of Morgan Baines. He frequently shares surprisingly detailed updates and theories with Sadie, blurring the lines of professional conduct and sparking suspicions about his true motives in the tight-knit island community.

Morgan is the vivacious and beautiful neighbor of the Foust family whose sudden, violent murder shatters the quiet of the Maine island. Rumors of her close connection to Will spark intense jealousy and speculation, making her death the focal point of the island's gossip and Sadie's frantic investigation.

Jeffrey is Morgan's husband, who finds himself thrust into the center of a murder investigation upon his return from Tokyo. Due to the brutal nature of his wife's death and rumors of his previous infidelities, he quickly becomes Sadie's primary suspect, despite his seemingly solid alibi.

The unnamed six-year-old daughter of Jeffrey and his ex-wife, Courtney. She is the center of a bitter custody dispute and suffers the immense trauma of finding her stepmother Morgan's murdered body.

Alice is Will's sister and Imogen's mother, who suffered from severe fibromyalgia. Her recent death by suicide serves as the catalyst for the Foust family's move to Maine, leaving behind a house filled with memories and a deeply traumatized teenage daughter.

Courtney is Jeffrey Baines's ex-wife and the biological mother of the six-year-old girl who found Morgan's body. She is locked in a bitter custody battle with Jeffrey, making her a person of interest in Sadie's amateur investigation.

Erin is Will's breathtakingly beautiful former fiancée who died tragically in a car accident decades ago. Her memory continues to cast a long shadow over Will and Sadie's marriage, feeding Sadie's deep-seated insecurities.

Mr. Nilsson is an elderly, frail neighbor of the Fousts. He becomes a critical figure in the murder investigation when he reportedly gives police a statement claiming he saw Sadie engaging in a physical altercation with Morgan shortly before her death.

Mrs. Nilsson is the Fousts' elderly neighbor who uses a wheelchair. Along with her husband, she observes the neighborhood's activities, adding to the claustrophobic atmosphere of the small island community.

Fake Mom is the moniker Mouse uses for her cruel and abusive new stepmother. She actively works to isolate Mouse from her father, creating a terrifying and oppressive domestic environment for the young girl.

Mouse's father is initially a loving and playful figure who shares cookies and games with his daughter. However, his frequent absences on business trips leave Mouse entirely at the mercy of his cruel new wife.

Carrie is a graduate student of Will's in Chicago. She becomes a target of Camille's calculating surveillance after Camille notices her spending time with Will at the university.