Kate is a 48-year-old mother of four living through the turbulent spring of 1969. She carries significant unaddressed guilt regarding her first husband's suicide, interpreting her son's draft into the Vietnam War as cosmic punishment. Though married to a kind second husband, Kate isolates herself emotionally and heavily relies on alcohol to suppress her daily anxieties.

Blair is a 24-year-old pregnant woman whose astrophysicist husband insists she quit her job and stay home. Feeling intellectually stifled and suspicious of his erratic behavior, she contemplates leaving him. She fixates on romanticized memories of her ex-boyfriend to escape her current domestic unhappiness.

Kirby is a 20-year-old college junior who projects a confident, politically active exterior but secretly harbors deep shame over a past relationship. She works the night shift at a hotel to distance herself from her mistakes. She genuinely wants to support civil rights and equality but frequently faces the harsh racial and gender realities of the era.

Jessie is a 13-year-old girl entering adolescence during a turbulent summer. She feels disconnected from her older siblings and emotionally neglected by her distracted mother. To regain a sense of control over her changing body and the confusing adult world, she begins shoplifting small items from peers and family members.

Tiger is an athletic 19-year-old drafted into the army to serve on the front lines. Before deploying, he lacks clear direction in life, having quit his passion for football. He writes letters home detailing the stark realities of war while carrying his grandfather's ring for luck.

David is a successful, level-headed lawyer and Kate's second husband. As a Jewish man, he faces quiet prejudice from the Nantucket elite and active disdain from his mother-in-law, Exalta. He deeply loves his wife but grows increasingly frustrated by her reliance on alcohol and refusal to leave her mother's shadow.

Exalta is a 75-year-old wealthy widow who fiercely upholds traditional social decorum. She dominates her daughter Kate and shows clear favoritism toward her late son-in-law, Wilder, over David. She demands strict adherence to appearances but occasionally reveals a softer side to her trusted employees.

Bill is the septuagenarian caretaker of All's Fair who maintains the property and provides a steady presence for the family. He allows his estranged daughter and grandson to stay in the adjacent cottage. He treats his employer, Exalta, with deep respect and loyalty.

Lorraine is Bill's estranged daughter who grew up alongside the Foley children. She abandoned her son, Pick, at a California commune. Her past reckless behavior and complicated history with the Foley family create lasting tension on the island.

Pick is Bill's 15-year-old grandson. Having spent his early years on a commune before his mother abandoned him, he possesses an independent, countercultural attitude. He works locally and seeks casual affection to fill the void left by his absent mother.

Angus is a brilliant astrophysicist whose demanding career at NASA and MIT keeps him away from his pregnant wife. He struggles with severe, unexplained bouts of sadness and acts secretively, causing Blair to believe he is unfaithful.

Joey is Angus's younger brother and Blair's former boyfriend. He lives a spontaneous, busy life in New York City. He maintains a flirtatious dynamic with Blair, presenting himself as a romantic alternative to his serious brother.

Darren is an affluent African American Harvard student working as a summer lifeguard on Martha's Vineyard. He quickly connects with Kirby over their shared antiwar sentiments, but he hesitates to integrate her into his family life due to racial tensions and his parents' protective expectations.

Patty is a would-be actress and Kirby's housemate. She becomes involved in an intense, dangerous relationship with a wealthy young man. She consistently ignores Kirby's warnings about her boyfriend's aggressive behavior.

Luke is a wealthy, privileged young man from New York City who summers on the island. He drinks heavily and exhibits volatile, controlling behavior toward Patty, expecting zero consequences for his actions.

Scottie is a police officer who previously arrested Kirby during a protest. He used his authority to initiate a clandestine relationship with her, deliberately hiding the fact that he was married and cutting contact when she needed help.

Wilder was Kate's charismatic first husband and a respected military veteran. Despite his public reputation, he suffered from bipolar disorder and engaged in numerous destructive affairs before his death by suicide.

Dr. Frazier is Darren's mother and a respected physician. She strongly disapproves of Darren's relationship with Kirby, wanting to protect her son's promising future from societal prejudice and scandal.

Garrison is a 19-year-old, privileged tennis instructor at the elite country club. He uses his position to act inappropriately toward young female students, trusting his status will shield him from consequences.

Bitsy is Kate's longtime friend who heavily drinks and engages in indiscreet affairs. She represents the hypocritical, status-obsessed social circle that Kate begins to outgrow.

Helen is Bitsy's daughter and one of Jessie's peers. Initially perceived as mean and unapproachable, Helen shares a vulnerable connection with Jessie after both girls face identical harassment at the tennis club.