A reclusive and emotionally withdrawn handyman, Lee Chandler, works as a janitor in Quincy, Massachusetts, living a life of routine solitude marked by brief, impersonal interactions. His life takes an abrupt turn when he receives news that his older brother Joe has died from a heart condition. Lee returns to his hometown of Manchester-by-the-Sea to handle his brother's affairs and discovers that Joe has made him the legal guardian of his teenage nephew, Patrick.

While trying to figure out his new responsibilities and grappling with the decision to become Patrick's guardian, Lee is forced to confront a tragic past that precipitated his departure from Manchester-by-the-Sea years earlier. As Lee struggles to deal with the present demands of caring for Patrick, his interactions with his ex-wife Randi and the townspeople, and the memories of his former life resurface, revealing the depths of his grief and the reasons behind his self-imposed exile.