Lee Chandler, portrayed by Casey Affleck, is a reserved handyman taking care of several elderly apartments in Quincy, Massachusetts. Under the supervision of his boss, Mr. Emery (Stephen Henderson), Lee struggles with a difficult relationship with the tenants, often responding to their disrespect with rudeness and hostility. Despite the attempts of patrons at a local bar to engage him, Lee shuns social interactions, preferring solitude and even picking fights with strangers who happen to glance his way.
When Lee learns from family friend George (C.J. Wilson) that his brother Joe, played by Kyle Chandler, has suffered a heart attack, he rushes to the hospital but is too late; Joe passes away before Lee arrives. In the wake of this tragedy, Lee informs Mr. Emery of his need for temporary leave and arranges for another handyman to cover his shifts. Acknowledging that Joe had been living with a serious heart condition, Lee reflects on Joe’s difficult marriage to Elise, who struggles with alcoholism and neglectful parenting.
Returning to his hometown of Manchester-by-the-Sea, Lee must break the news to Joe’s teenage son, Patrick, played by Lucas Hedges. Unbeknownst to him, while arranging the funeral, Lee discovers that Joe designated him as Patrick's guardian, along with leaving $5,000 for Lee to relocate back to Manchester. Initially assuming that their uncle Donny would take custody, Lee grapples with the responsibility that Joe intended for him. Torn between not wanting to return to his painful past and refusing to let Patrick go back to his mother, Lee compromises and decides to stay until the school year ends.
Flashbacks reveal a happier time when Lee was married to Randi, portrayed by Michelle Williams, and they raised their three children. This once joyful life takes a dark turn when a tragic accident results in a fire that takes the lives of their children, leading to Lee's emotional unraveling, a failed suicide attempt, and eventual divorce. The weight of guilt and loss has left Lee a bitter and isolated shell of his former self.
As he navigates the complexities of grief and guardianship, Lee's strained relationship with Patrick begins to heal. When Patrick suffers a panic attack, Lee is there to support him, and they slowly rebuild their bond. However, tension continues to simmer as Lee's desire to leave for Boston clashes with Patrick's rootedness in their community. Patrick's mother Elise attempts to reclaim her role in his life, adding complications to Patrick's emotional turmoil.
In encounters with Randi, whom Lee tries to avoid despite her attempts at reconciliation, the pain of loss and regret surfaces anew. After a brutal emotional breakdown in a bar, Lee finds himself recovering at George's home, wrestling with his past and the future he is trying to escape.
Eventually, Lee decides to arrange for George and his wife to adopt Patrick, allowing him to remain in the town that is still so vital to his heart, all while he finds a job in Boston. The story culminates in a bittersweet fishing trip on Joe's refurbished boat, symbolizing both a connection to the past and the tentative steps toward healing their collective grief. In the end, Lee realizes the importance of the bonds that still connect him to his family and town, which will forever be part of his journey toward redemption.
