Delusion

When people experience a life-shaking event, whether religious conversion, near death experiences, or other tumults of life, they often come out changed to the point where you wonder if they are the same person. Suppose, in at least some cases, they aren't the same person. Demonic possession has long been studied to either debunk or prove same. Those saying their new selves are higher spirits are treated with less by way of exorcism, but more by way of therapy...often forced.

Two university students for a senior thesis are given one of the latter to study. When reports by professionals begin to fall short, they seek out the victim's sister. Through her help, both students are afforded a face to face meeting with the victim himself.