Lena Headey, of Game of Thrones fame, as Mary Sandin is decent. Her character isn't deeply defined. Do not use this film as a testament to her ability. She is much better than this.
Don't forget, this is an independent film with a small budget even by independent movie standards. Many people are criticizing it as though it isn't; which is only a testament to how great it really is. I suspect the sequel, because there will most definitely be one, will be much better.