Lee (Casey Affleck) is a depressed repairman. After learning of the death of his brother Joey (Kyle Chandler), Lee returned to his hometown, Manchester by the Sea, to deal with Joey's funeral. According to Joey's will, Lee will become the guardian of Joey's son Patrick (Lucas Hedges). Lee plans to bring Patrick back to Boston, but it is obvious that Patrick is unwilling to leave his hometown and friends, but Lee is unwilling to stay in this sad place for a long time. It turns out that Manchester is buried with Lee's desperate memories. His negligence caused three children to die in the sea of fire, and his wife Randy (Michelle Williams) also left him because of this. Returning to his hometown this time, Lee once again met Randy, who had remarried and was about to become a mother. At the same time, Patrick's long-lost mother Alice (Gretchen Mol) also contacted Patrick, hoping that he could become a member of her new family.