Many years ago, I watched "Mystic Pizza" so I assumed "Mystic River" took place in a southern Connecticut port town. But no, it's Boston based. A second level of confusion resulted in the fact that one character is named "Sean" but not the character played by Sean Penn, who is "Jimmy". Just as I can't keep track of who has the ball when the Vikings play Green Bay, I had trouble keeping the names straight, except for "Dave". And I did not realize this was a Clint Eastwood-directed film until the end, when the credits rolled.
This was one long, slow-paced, dark movie. I don't think there was a moment of levity in the whole thing. And a certain amount of knowledge had to be inferred from the dialog. Fortunately, my daughter was on hand to keep me up to speed.
The basic story line is, three eleven-year-old boys are deeply affected by what happens to one of them, Dave. Flash forward 25 years, and we see that one is a cop and one is a petty thief turned local boss of his own little fiefdom, while damaged Dave acts kind of dumb but is smarter than we think. Jimmy's nineteen-year-old daughter is brutally murdered, and Sean the cop works the case. He and his partner focus on one likely candidate after another, but by the time they nab the perpetrator, one of their suspects becomes fish food.
This movie reminds me of "Unforgiven" - another Clint Eastwood movie - which I "got" but could not explain. Things are not tied up in a neat little bundle in the end. If you can stand that kind of uncertainty, this may be a movie for you.