 Riker is injured on an away mission and is infected with a "virus" that binds to his nerves. The virus quickly grows and he is close to death and falls into a coma. In order to keep him alive Doctor Pulaski stimulates his brain and Riker starts dreaming (flashbacks) about happy times from the previous shows. Troi and Doctor Pulaski quickly discover that the type of brain stimulation affects the emotions of the dream which in turn affects the virus and as they try stimulating different parts of the brain the result is different kinds of flashbacks and affects on the virus. They finally find that the fear emotion kills the virus and they stimulate the fear center and the virus is killed.