Julian is a 75-year-old former portrait artist who lives in a London cottage he purchased in 1961. Following the loss of his partner, Mary, he abandons his previously vibrant social life and withdraws into complete isolation. He writes his honest, lonely feelings in a notebook and leaves it in a local coffee shop, hoping to inspire a chain of genuine human connections.

Monica is a former corporate lawyer who used an inheritance to open her own independent coffee shop, Monica’s Café. While she functions as a highly organized and efficient boss, she silently worries about her business finances and her profound fear of remaining single and childless. When she finds Julian's notebook, she writes her own vulnerable entry before leaving it at a wine bar across the street.

Timothy Hazard Ford is a former estate agent battling heavy substance addiction. After finding the green notebook in a bar and reading Monica's entry, he realizes she is the same woman he recently insulted on the street. He abruptly snorts cocaine using his resignation letter, destroys his phone to cut off his drug dealer, and travels to a Thai island to attempt sobriety.

Riley is a handsome, relaxed Australian tourist traveling through England. He discovers the green notebook hidden in his backpack during his flight and decides to visit Monica's Café to meet the people he read about. He quickly develops an attraction to Monica but keeps his prior knowledge of the notebook a secret from her.

Alice is a 26-year-old mother who runs a highly successful Instagram account focused on her seemingly perfect domestic lifestyle. In reality, she feels exhausted by her infant and entirely unsupported by her husband. She relies heavily on social media validation to cope with her feelings of inadequacy and isolation.

Max is Alice's husband and the father of their baby, Bunty. He struggles to adjust to the changes that parenthood has brought to his marriage and home life. Rather than supporting his wife through her anxieties, he becomes increasingly distant and critical of her obsession with social media.

Betty is an older Chinese woman who works as a tai chi instructor in London. She becomes a regular fixture in Julian's life and eventually joins his art classes at Monica's Café, bringing her own family concerns to the newly formed friend group.

Baz is Betty Wu's grandson. He carefully manages the information he shares with his grandmother regarding his romantic relationship, fearing her traditional judgment.

Benji is Baz's partner. He attends social events with the café group and supports Baz while attempting to handle the complex dynamics of Baz's relationship with his grandmother, Betty.

Lizzie is a volunteer at the Mummy's Little Helper charity center. She is a nosy but well-intentioned woman who inserts herself into the lives of the notebook participants after discovering the journal at the center.

Bunty is Alice and Max's young baby. Her constant needs and inability to sleep through the night cause severe exhaustion for her mother, acting as a source of immense domestic stress.

Mary is Julian's former partner. Her absence in Julian's life leaves a massive emotional void, plunging him into a deep depression and prompting him to start "The Authenticity Project" to cope with his profound isolation.