As far as the Vengeance Trilogy goes...OLDBOY is the best, followed by SYMPATHY FOR MR VENGEANCE and this one third. The story was too slow and the vengeance was weak as fuck. Visually though the film was great to look at!
The story jumps around back and forth and all over the joint, so I'll just give you the simple version and say a woman served 13 years in prison for the murder of a child she didn't kill. Now she's out and she wants revenge. She gets it, but it slooooowly. At least in SFMV you had a bunch of stabbing and that great baseball bat the the skull scene, in OLDBOY you had the tooth-pulling and the legendary hammer scene, but in SFLV there's really nothing that stands out. It's a good movie, but I found it overly boring and nowhere near as exciting as I had expected from Chan-wook Park.
Good film and definitely worth watch, but I found it to be a minor drag. Honestly the highlight of the entire movie was seeing the brief cameo by Kang-ho Song and that other dude, the guy who farted on his mother in THIRST. Check out Park's other stuff first.
Part 1 - Sympathy for Mr. Vengeance
Part 2 - Oldboy