It's hard to say much about Martyrs without giving away too much information. Trust me when I say, this is a movie you want to go in with knowing as little as possible. Here's the synopsis from IMDB:
A young woman's quest for revenge against the people who kidnapped and tormented her as a child leads her and a friend, who is also a victim of child abuse, on a terrifying journey into a living hell of depravity.
Martyrs is compared to other French horror films like Inside because of its graphic violence, but Martyrs is very different. It is violent, shocking, graphic but it's a very divisive, leaving viewers with a love it or hate it response. Some have called it "the new yard stick against which all forms of extreme genre films should be measured against." So of course, an American remake is on the way.