1 Rose's family kidnaps black people
Rose's family kidnaps black people then later sells their bodies to rich whites after putting them through a surgery. The surgery gives control of the body of the black person to the white person who bought it.
2 The surgery gives the person a new body
The surgery gives the white person the body of the black person. It's a brain swap operation. The white person embodies the new black body and gets almost full control over it as if it's their own body.
3 The black person remains conscious
After the surgery, the black person whose body was taken remains conscious. They see everything from a distance just like when Chris sank in his chair and saw his body from a distance but become unable to control the body.
4 Logan was given Dre's body
Dre, the guy who was kidnapped in the beginning of the movie went through the surgery. Dre's body was given to Logan, the wife of the white woman who showed up with him later in the party.
5 The camera flash awakens the black person
While the white person has total control over the body, a camera's flash can bring the black person some of their consciousness. This is why Logan warned Chris and told him to get out as soon as he saw the flash. (See The Usual Suspects (1995) Movie Explained)
6 Missy hypnotizes people to paralyze them
Missy hypnotizes victims in order to paralyze them and get them ready for the surgery.
7 Georgina is Rose's grandma
Georgina became Rose's grandma after the black woman was kidnapped by the family then sent through surgery. (See The Suffering (2016) Movie Explained)
8 Walter is Rose's grandpa
Walter became Rose's Grandpa after he took the body of the black person through the brain transplant surgery.
9 The black person inside Walter awakened
When Walter was about to kill Chris, he used the mobile phone's camera to make Walter see the flash. The black person in Walter's body woke up and killed Rose. He then committed suicide because he couldn't stand living in that state anymore.