As far as plot goes I can't say much without giving anything away, so I'll keep it simple. Lucie (Mylene Jampanoi) escapes from a warehouse where she had been tortured for quite some time. She is taken to a hospital where she meets Anna (Morjana Alaoui). The two almost immediately become friends. 15 years later, the now seriously unhinged Lucie sets out on a quest for revenge which will have horrifying consequences for both girls. This is a French torture-porn film (It's not as much about revenge as you would think). It's also so much more than that. It may be the most relentless, IN-YOUR-FACE, graphic, unpleasant, visceral, non-stop, bloody, brutal, emotionally affecting, and profoundly disturbing horror film I have ever seen. What it sets out to do, it does extremely well, not to mention it throws in some twists too (which was appreciated), and it actually has meaning. Martyrs is an unforgettable film, you won't be able to un-see what is seen here. It's certainly not for everyone, and it is definitely an incredibly challenging, difficult watch. Personally, I wasn't crazy about it as it really wasn't made for me, though I can respect that it doesn't hold back. This is the kind of film Eli Roth wishes he could make. This is torture porn done right.
If you are at all interested in Martyrs, stay FAR AWAY from the American remake trailer, as it gives just about everything important about the original away.