Seventeen-year-old GREGORY WILES is working at his computer and chatting with a pal on the phone, when a mysterious window pops up prompting him to click a button. Strange images begin to flash before his eyes, and the boy is drawn in, seemingly mesmerized. Then something begins to protrude from the middle of the screen and impossibly takes the form of a hand. The unearthly appendage slowly reaches forward and without warning grasps the boy's face...
At her apartment, Olivia enjoys a game of "Operation" with her niece, Ella. But the lighthearted moment is interrupted when Rachel catches Ella in a lie: she hasn't brushed her teeth this morning and Rachel isn't crazy about being lied to.
The Harvard lab. While Walter pontificates about Darwin's theory of human sexuality, Peter shuffles through the mail and finds a letter that apparently disturbs him. He throws it in the trashcan, but not before Astrid notices the look on his face. Olivia calls and says she needs Peter and Walter to meet her outside because they have a new case. As the two stroll out, Astrid retrieves the letter from the trashcan and reads it.
Charlie and Olivia talk to Gregory Wiles's parents, trying to find a motive for someone to target Gregory. Both parents are at a loss, but they admit that their son spent much of his time on the computer chatting with his friend LUKE DEMPSEY.
In the lab, Walter drills a hole in Gregory's head and drains his cranial cavity. Walter has determined that Gregory's entire brain was liquefied, but he's not sure how. He hypothesizes that Gregory may have contracted an advanced form of syphilis that cooked his brain. Peter says that if he caught something from a girl or took some kind of drug, there might be a record of it on his hard drive. Astrid is already on it, but she says the hard drive's platters have been fused together.
Olivia pays Luke Dempsey a visit. Luke is clearly shaken by the news of his friend's death. He says the two of them were talking on the night of Gregory's death, but Gregory had stopped responding. He also tells Olivia that he knew Gregory because their fathers had worked together. As she's leaving, Olivia gets a call from Peter to report another death that appears to be related.
A car dealership. Broyles briefs the team on the death of the dealership's general manager, whose head rests in a puddle of melted brain matter. Walter quickly deduces that the cause of death is the same as Gregory's.
In the lab, Astrid finds that the new victim's hard-drive has been damaged in the same manner as Gregory's. Although unable to recover much data, she has determined that both victims downloaded an enormous file just before their computers crashed. The conversation is interrupted by the unfamiliar sound of the laboratory's old rotary phone ringing. Peter answers but hangs up quickly. He claims the call is a wrong number, but his demeanor says otherwise. He leaves with the hard-drive, saying he knows a guy who might be able to recover more data from it. Once they're alone, Astrid tells Olivia she thinks she knows who called and shows her the letter Peter threw away earlier.
Peter pays a visit to AKIM, an old acquaintance, and persuades him to help identify the mysterious program downloaded by both victims. Akim leads Peter into a back room, where he has a fully functioning high-tech computer lab.
Meanwhile, at the FBI office, Sanford Harris trashes Olivia's investigative approach, saying the case should be turned over to the CDC. Harris threatens to shut down her investigation if the case isn't solved in twelve hours.
In the computer lab, Akim tells Peter that the program in question is incredibly complex, and some of it is not even computer code. At the FBI, Olivia searches the FBI database and cross-references medical records for the two victims. As Olivia awaits the results, Akim says he can't find the source of the program -- but then realizes the program is being downloaded at this very moment. He narrows down the destination, which Peter instantly recognizes as Olivia's apartment. Meanwhile, at Olivia's apartment, Ella plays a game on a laptop.
After getting Peter's frantic call, Olivia races to her apartment. She calls her sister, but Rachel can't hear the phone. While Ella plays her computer game, a familiar mysterious window pops up. She clicks it, and the same series of images begins to play. Ella appears drowsy, hypnotized. Outside, Olivia jumps the curb with her car, squeals to a stop, and dashes into the apartment. Peter is right behind her. Olivia pulls the laptop away from Ella's hands and wakes her from her trance.
Later. Olivia tells Charlie about the video on the computer that hypnotized Ella, and Charlie agrees to call computer forensics. Meanwhile, Peter bonds with Ella and Rachel. Ella says there was a hand reaching out of the computer. When Olivia inspects the computer, she sees the light next to the video camera is on. She looks right into the camera...
And her face appears on the screen of a man, BRIAN DEMPSEY, who sits behind an array of high-tech equipment in an undisclosed location. He talks to the screen, telling Olivia's image that she'd better back off or she won't be so lucky next time. Just then a proximity alarm goes off and someone enters the building. It's Luke Dempsey, Brian's son, who came by to bring his father some food and to check up on him. Luke spots a pillow and some blankets and realizes his father has been sleeping at the warehouse. Brian says he's been busy working on a new computer program that he thinks a lot of people will be very impressed with.
At the FBI, Olivia regroups with Broyles and Peter. Olivia theorizes that whoever is responsible for these deaths was watching her through the computer's video camera. Broyles suggests that the killer may be aware that Olivia is on the case and hacked into their system to track her. Olivia says she has yet to find any connection between the victims, which makes finding a motive difficult.
Evanston, Illinois. A woman comes home to find her husband, MARK ROSENTHAL, dead in front of his computer. His liquefied brains have spilled out of his ears and nose onto his shirt.
Harvard. Olivia receives the call from Charlie informing her about the next victim. As Olivia deals with the call, Peter spots an older woman staring in his direction. He walks up to the woman, who turns out to be JESSICA WARREN. Peter says that he read her letter and understands why she wishes to see his father, but Walter isn't ready, and meeting him won't bring her daughter back. When the woman continues to press, Peter cuts her off and says it's not going to happen.
In the lab, Walter explains how he thinks the victims died: the program created audiovisual stimuli that amplified the victims' neural impulses and trapped their brains in an endless loop -- much like a computer virus that attacks people.
Later, Olivia confronts Peter about Jessica Warren. Olivia knows she's the mother of the lab assistant who was killed in the same fire that put Walter in the mental institution nearly twenty years ago. Olivia thinks Peter should let Walter and Mrs. Warren talk, but Peter vehemently disagrees. Astrid enters and presents Olivia with background on the latest victim, and they finally make a connection: the latest victim recently married Marion Dempsey, Luke Dempsey's mother. But why would Luke kill his own friend Gregory? Then Olivia remembers that Luke's and Gregory's fathers had worked together.
A short investigation reveals that Gregory Wiles' father used to be Paul Dempsey's boss, but fired him. Paul is apparently going after people who hurt him by targeting their loved-ones.
FBI Office. Olivia, Charlie and Harris stand behind a one-way mirror, looking at Luke Dempsey in the interrogation room. Olivia suspects Luke knows there's something wrong with his father, and that if they interrogate him aggressively he may shut down or ask for a lawyer. She wants to wait for the right moment, but Harris disagrees, telling her to get in the room and break him.
During interrogation, Luke says it's been a few weeks since he's seen his father. Olivia and Charlie aren't getting what they need, so they show him crime scene photos of Mark Rosenthal and of his friend, Gregory Wiles. When they explain that the only connection is his father, Luke shuts down and asks for a lawyer. Outside the room, Olivia tells Charlie to wait five minutes and then release Luke.
Once Luke is released, Olivia listens in when Luke calls a cab to pick him up. She gets the destination address. On the way there, Olivia and Peter argue again about Jessica Warren. Olivia thinks Peter is doing his father a disservice by not being forthright with him. Peter says it's none of her business. When they arrive at the warehouse, Olivia tells Peter to wait in the car.
Back at the FBI, Harris discovers Luke has been released and that Olivia is gone. He orders a trace on her vehicle.
Inside the warehouse, Luke confronts his father about the killings. Brian says he couldn't let himself be walked on. Then the proximity alarm goes off, and Brian he sees Olivia on his monitors. He orders Luke to leave. Olivia moves silently through the building, unaware she's being watched. As she reaches the den of the killer, all of his monitors light up with the familiar flashing images. Brian appears from behind her, puts a gun to her head, and disarms her. He holds her at bay with one gun while pointing the other under his chin. Olivia tries to convince him not to end his life. But Brian turns his attention to the screens so he can experience his creation.
Peter hears sirens closing in and goes inside the building. He is immediately surprised by Luke, who wields a baseball bat. As Peter tries to talk Luke out of doing anything stupid, a gunshot rings out. Peter and Luke run toward the shot to discover that Brian has killed himself. A distraught Luke tries to escape, only to be greeted by a swarm of agents with guns drawn.
In the car, Peter asks Olivia why Luke would protect a murderer. Olivia gives the obvious answer: because it was his father.
At the FBI, Harris tells Broyles that Dunham violated his direct orders by releasing Luke. Broyles disagrees, saying Olivia closed the case in spite of Harris's obstructions. Broyles tells Harris that if he continues his personal vendetta against Olivia, he will put his career on the line to protect her.
In the lab, Peter has changed his mind. He tells Walter there's a woman who wants to see him -- but that he can end the conversation any time he wants. Jessica says her daughter's name was Carla Warren, and that lands hard on Walter. The sad look on his face says he remembers her. Jessica wants to know if there's anything Walter can tell her about Carla, since he was the last person to see her alive. Walter says he remembers she had a wonderful smile. The two embrace as Peter looks on.
Back at home, Olivia talks with Rachel. The doorbell rings, and it's Peter. He tells Olivia that she was right about Walter... and more importantly, that he's grateful to Olivia for bringing Walter back into his life. He was scared that letting Walter talk to Jessica Warren might compromise that. As Rachel comes out and greets Peter, there's just the hint of chemistry between them, and Olivia's expression shows she notices. But Peter simply says good-night and goes on his way.