Many consider Manchester by the Sea to be 'Oscar bait' in the sense that it tries so hard to be a film about performances rather than narrative, and it's quite hard to argue against that; it's best to simply acknowledge it and appreciate the fact that it's a very different film. It's one that explores the grieving process, how a regular person may react when faced with the inevitability of loss and death.
Lee Chandler (Casey Affleck) is placed as the sole guardian of his nephew after the passing of his older brother Joe, he returns to his hometown, Manchester-by-the-Sea, to be with his family during the difficult time. Lee isn't exactly strong, though: he's a simple janitor with some obvious emotional problems given his hard past in which negligence during intoxication caused a house fire, killing his three children. Lee, found to be innocent and under no criminal charges, pulls a gun from an officer's holster and attempts to commit suicide.
He remains reluctant to become the guardian of his nephew, he believes he is too destructive to be given a life with responsibilities. He thinks being alone is best for him and everyone else, although he doesn't really have a choice in this situation. The hardships of life and the losses they've both faced eventually cause the two to bond, as they establish a connection and realise that they can both grieve together as they focus on their futures.
Manchester by the Sea reflects the working class during difficult times, its performances display a depressing amount of realism which is often ignored in the world of cinema. It's sad. It's a little slow. It's beautiful. It's Oscar bait.