Louise is a single mother living in London who works as a receptionist in a psychiatry practice. She struggles with low self-confidence, loneliness, and recurring night terrors about losing her son. Her routine life becomes complicated when she unwittingly befriends her new boss's wife while simultaneously pursuing a romantic relationship with him.

David is a psychiatrist with a charming exterior and a specialty in addiction treatment. He grew up on a farm and has known his wife since childhood, having saved her from a devastating fire. In the present, he appears burdened by his marriage, strictly controlling his wife's finances and movements while secretly self-medicating with alcohol.

Adele is a wealthy, beautiful woman who inherited her family's Scottish estate after her parents died in a fire. She presents a sweet, fragile exterior to her new friend Louise but harbors intense, obsessive love for her husband. She possesses a history of psychiatric treatment and utilizes lucid dreaming techniques to manage her night terrors.

Rob is a patient at the Westlands therapy center, placed there to manage his heroin addiction and night terrors. Coming from a broken home, he is cynical about therapy but forms an immediate attachment to Adele. His thoughts and past experiences survive in the present day through a detailed notebook he kept during his treatment.

Adam is Louise's six-year-old son who splits his time between his mother and his father. He represents the only uncomplicated, purely joyful part of Louise's life, grounding her when her adult relationships become chaotic.

Sophie is Louise's worldly, practical friend who frequently engages in extramarital affairs. She provides blunt, unfiltered advice to Louise regarding her complicated involvement with her boss and his wife.

Ian is Louise's ex-husband who has remarried and is expecting a new baby with his wife. He maintains a functional co-parenting relationship with Louise but occasionally causes her anxiety regarding custody time.

Anthony is a young man struggling with heroin addiction who seeks treatment from David. His presence inadvertently tangles him in David and Adele's marital discord when he visits their home and misinterprets Adele's situation.

Dr. Sykes is the senior partner at the psychiatric clinic where David and Louise work. He is easily charmed by Adele but maintains professional authority over the office operations and employment matters.

Marianne is a woman from David and Adele's previous town who once served as a platonic confidante for David. She experienced Adele's extreme behavior firsthand, prompting her to warn others away from the couple.

Sue is a receptionist at the psychiatry clinic who works alongside Louise. She is highly observant of office dynamics and quickly notices changes in Louise's behavior and appearance.