Roy McComb is an ordinary office worker, although tortured with images of disasters and terrorist attacks, turning out to be accurate precognitions of actual events. In order to deal with the images harassing him, he finds himself forced to draw - or sometimes construct models of - the disasters. Originally trying to find the hands of God, or even the Devil, behind his precognitions, he eventually isn't able to find any other explanation, than that he has gone insane.
Following the bus attack killing several civilians, as well as DEA agent Evelina Mendoza, the FBI learns about Roy's paintings, and take him in for interrogation. Clearing him from all criminal suspicions, although acknowledging some kind of link with the people responsible for several terrorist attacks, including the one on the bus, Roy is handed over to Dr. Walter Bishop and agent Olivia Dunham.
As it turns out, Roy was an old "patient" of Walter's, from the time prior to his institutionalization. Roy had allowed the doctor to conduct what was supposed to be harmless experiments on him, in order to earn money for his education. As a result Roy has become a radio receiver for messages sent over what's referred to by Walter as "a ghost network" - which by pure coincidence happens to cause Roy to hear the phone calls of the people responsible for the terrorist attacks, and thus causing the images in his head.
After being able to track down the terrorists, thanks to the messages received by Roy, Walter "cures" him, effectively freeing him from any further images or other precognitions. ("The Ghost Network")