If you didn't read the book:

Nick Dunne (Ben Affleck) leaves his rural Missouri cul de sac one sunny July day and heads to visit his twin sister, Margo (Carrie Coon), at a neighborhood  bar they co-own. There, Nick receives a call from a neighbor letting him know—we assume—that his front door is open and his cat has gotten out. When Nick returns to retrieve the animal, something is not right: A coffee table is overturned and a chair is askew in a small sitting area; and his wife, Amy (Rosamund Pike), is gone.

From there, things get weird. Suddenly, Nick's mediocre marriage with a callous and dissatisfied wife are thrown into the national spotlight, as the world watches locals and law enforcement search for Amy Dunne. Amy is portrayed as a martyr, who moved from Manhattan to stay by her husband's side as he cared for his dying mother. Nick, meanwhile, is a leaching, philandering loser, who--it seems--murdered his sainted bride.

All signs point to Nick as the perpetrator; and, in spite of detective Rhonda Boney (Kim Dickens) and high-priced attorney Tanner Bolt (Tyler Perry) believing his innocence, it seems only Amy—alive or dead—can exonerate her husband.

If you read the book:

The film follows the book with pinpoint accuracy. There are a few extremely minor plot points, mentioned in blogs, that differ. I will say this, though: The dynamic of the film is slightly different than the novel. The reader is more sympathetic to Nick's narration. In the novel, he comes across as more of a victim, a bumbling pawn in a psychotic game. In the movie, you really don't care if he gets pinned with the crime or not—or if you do, it's only out of a sense of justice, not caring.

My only criticism is that the film could have been an opportunity to enhance the novel, not simply recreate it on screen. I was hoping for a little gift for the readers out there. That being said, it is a very good book—and an equally compelling film.

Either way:

The acting here is spot-on. Affleck is the perfect Nick—good looking and confident, but not as smart as he should be; and Rosamund Pike is chilling as the missing Amy. David Fincher's direction, as usual, is flawless. All in all, it's an exceptional thriller. It's an 8.