A doctor named Bill Rand is walking home one night when a living shadow man beats him up and stabs him to death.
Next morning, Jess and Nathan meet up and talk. Their talk is put on hold when Audrey calls him about the murder so he goes to the site. Nathan admits that he's still very nervous about his relationship with Jess. They examine the body and figure that he was beaten up by something with superhuman strength. Bill worked at Hessberg Medical Center, where Jess has recently gotten a job as a caretaker.
As they enter the medical center, Nathan teases Audrey about how she can't remember the other officers' names. Indignant, she tries to list their names, but gets a lot of them wrong. Dr. Wilson says that some people claim that the Dark Man has been killing doctors and patients. Jess is doing grief counseling. Thornton Aarons, a blind man, talks about his dead wife Sarabeth. The other patients also claim the Dark Man was responsible for the killings.
They go back to the station to go over their limited information and Jess visits them. She kisses Nathan, who nervously backs away. Nathan asks her to stay away from the center, as it might get dangerous, but she refuses, dismissing the Dark Man as a superstition.
Dr. Wilson is at home talking with her husband when he leaves the room to cook. The shadow figure enters, flat against the wall, but becomes solid to impale both her and her couch with a shadow sword.
Nathan and Audrey examine the murder site and are baffled at how it could have happened, noting the impossible angle and how the wall is undamaged even though the couch was right next to it. They question several patients, but their descriptions of the Dark Man are all different (one describes him as tall, another short, etc).
They visit the Teagues and check their archives for similar incidents, but find nothing. Vince and Dave say that they've never heard of the Dark Man. Audrey suggests Nathan take a break and go out with Jess, but he insists on staying on the case. She demands to know what is the matter with him, and he explains that he doesn't think it can work out because he can't feel anything. Audrey orders him to at least try, and he finally agrees.
Later, the families of the Dark Man's victims are angry at Audrey for having their bodies exhumed. She replies that she's sorry but she is trying to find out how the Dark Man operates.
At Jess' house, she and Nathan have wine. He talks about how he can't feel. She kisses him, but he doesn't feel it. She tries again, urging him to feel it in his heart. He kisses her back.
The Teagues visit Audrey at the station when the Dark Man enters and attacks them with his sword. Dave takes a few pictures of him and the flashes briefly stun him, but he presses his attack. Audrey realizes that he can't exist without light and starts turning off all the lights, but the Dark Man guards the remaining light source.
Nathan and Jess are about to have sex when his cellphone rings. She says to ignore it, but he answers. Audrey tells him what is happening and he rushes to the station. He shoots out the remaining light, and the Dark Man disappears. They realize everyone described him differently because he changed shape depending on the light. Audrey apologizes for ruining Nathan's date.
The next morning, Jess talks with Thornton and he talks about how Sarabeth died when her chemotherapy failed.
Eleanor goes over the medical center's records and finds that none of the patients were properly given chemotherapy.
Jess goes over Thornton's medicine cabinet and finds a lot of unused pill bottles.
Nathan and Audrey look over Dave's photos and find that each flash destroyed a small part of the Dark Man's body. They then find evidence that Rand and Wilson had been selling chemotherapy drugs for money instead of administering them to their patients.
Thornton tells Jess that he keeps Sarabeth's old medications for their sentimental value. She leaves, taking two of the bottles.
The officers figure that the Dark Man killed the two doctors after he learned of their crime. Audrey wonders why he attacked her. Jess calls about the medication and says she has figured on her own that the patients were not really getting chemotherapy drugs. Nathan figures that she is in danger and rushes over to her house.
Thornton realizes that Jess stole two bottles and becomes enraged.
The Dark Man enters Jess' house and attacks her. Nathan and Audrey show up and the Dark Man leaves. Jess was knocked unconscious.
Nathan is very upset about his girlfriend almost getting killed, and Audrey urges him to calm down and look at the case rationally. Their only lead now is Thornton. They visit him in the medical center and find that he doesn't have a shadow. They realize that the Dark Man is his shadow, and that the Dark Man's sword is the shadow of his cane.
They find that Thornton doesn't know about the Dark Man, but Audrey gets an idea, urging Nathan to not do anything stupid. She tells Thornton that the people responsible for all the deaths are being held at the station's interrogation room, then drives back there.
Audrey puts two target silhouettes in chairs and waits. Nathan tells Thornton what they know and Thornton realizes with dread that the Dark Man targets people after he becomes angry at them. He begs forgiveness for the attack on Jess and asks Nathan to kill him, hoping the Dark Man will die as well. Nathan says he's thinking about it.
The Dark Man appears in the interrogation room and attacks the silhouettes. Audrey shines some heavy-duty lights on him and he disintegrates. Thornton's shadow reappears. Nathan says that he promised Audrey he wouldn't kill him.
The next day, Nathan comes to see Jess, but she says that she's breaking up with him and moving to Montreal. She can't handle the Troubles anymore after she almost got killed. She kisses him goodbye and leaves.
Audrey and Nathan put a sign on a door that says that the occupant must not be disturbed. By keeping Thornton in a pitch black environment, the Dark Man cannot exist, as without light, there can't be any shadows. Nathan is depressed and tells Audrey about the break up. Audrey says that she's there for him if he ever needs a shoulder to lean on and kisses him on the cheek. Nathan goes wide-eyed. As Audrey walks away, Nathan stares at her and puts his hand over the spot she kissed. He felt it