In a funny way, Mystic River is like a real-world version of Stephen King's It set in real life. The basic set-up, themes, and narrative structure are largely the same. But instead of an evil spider from outer space, the final boss monster is the characters' own weaknesses.
Mystic River begins with three boys: Sean Devine, Jimmy Marcus, and Dave Boyle. They live in a white ghetto outside Boston, and in the way of children, they are not exactly friends, even though they spend all their time together. One day a car pulls up with two men in it. These violent pedophiles pretend to be cops, and they steal Dave Boyle, and they keep him imprisoned in a basement for four days until he finally manages to escape.
Dave Boyle is never quite the same. But then again, neither are Sean and Jimmy.
As adults, Dave is kind of a loser. Sean has become a wildly successful cop, and Jimmy was a wildly successful criminal until he became a father and went straight. All three men are still reacting to that early formative event: Sean by protecting the weak, and Jimmy by becoming bad enough to protect himself from anything.
One night Jimmy's daughter Katie goes out drinking with her friends. She doesn't come back alive. Although the story's catalyst is a dead girl, it's interesting to contrast this book with the millions of other crime novels that rely on naked dead ladies as props. Katie is allowed to be human, a real character, unlike all the other sad naked dead ladies who get shipped off to the fictional morgue for analysis, unloved and unmourned.
If I have any complaint about the book, it's that it presents the victims of sexual abuse as being unfixably broken. This is both wrong and needlessly fatalistic, and yet it is a commonly-held belief, which is why so many abuse victims commit suicide*. I feel like not only did Sean and Jimmy not give Dave a chance, but neither did Dennis Lehane.
It's interesting to compare Mystic River with Shutter Island. Both books are steeped in grief and loss, and both books feature a surprise twist. Although in the case of Shutter Island, the twist is bigger - and less justified - than in Mystic River. (Which isn't to say the twist in Mystic River isn't implausible. It relies on a surprising coincidence, and on the investigating officers overlooking a 3,200-pound item of evidence.)
The big difference between the two books is that Mystic River introduces the twist slowly throughout the first half of the book. As a reader, your doubts pile up, just as they do for the detectives investigating the case. And in the end, the surprising coincidence makes sense. It's not so much a "surprising coincidence" as it is cause and effect.
* The suicide rate among survivors of sexual abuse is up to 14 times higher.