MacGyver and Pete are participating in a stakeout with the police, watching a crack house. A prostitute arrives and they wait for word from Murphy, the cop in charge. The prostitute leaves and they call in MacGyver, who goes around to the side of the house and meets with another cop, Sweeney. He sets up a series of advanced explosive charges on the gate covering door but one of them fails to activate. MacGyver and Sweeney break the gate down and another cop, Copper, breaks down the door with a battering ram. Sweeney stops one dealer while the other locks himself in the bathroom to flush the dope down the toilet. MacGyver and Cooper break in and capture the man, but they lose the dope. Fortunately, Murphy put another cop, Wyatt, beneath the toilet to grab the evidence. One more dealer tries to destroy more crack in the garbage disposal but Murphy arrives and shoots out the disposal before the evidence is destroyed.
Murphy is less than thrilled with the new technology provided by the Phoenix Foundation via a five-year contract. MacGyver points out that the public uproar over the tank she used is what forced them to abandon the tank in the first place. He asks for a second chance but Murphy refuses, so he plans to go ahead without her cooperation.
At the state mental hospital in Alameda, an older prisoner, Dr. Zito, talks to another prisoner, Eric Cross, and assures him that "she" will come and will be Eric's Messenger. Zito promises Eric a breakthrough and that he'll make his way to Valhalla. He tells Eric to lie down and then calls a nurse as Eric starts convulsing. The nurses conclude that Eric is having a heart attack and take him to the infirmary. Once he's alone with a male nurse, Eric revives and strangles the man. In his cell, Zito smiles in satisfaction as the alarm goes off.
In the police station's basement, MacGyver tests the Phoenix door-busting device but fails to get it to work. The station captain, Walker, informs them that a serial killer is on the loose and Pete and MacGyver are on the task force. When Murphy gets word, she appears nervous. The team goes over the file on Eric Close, the "Street Stalker." Eric stalks "loose women" and then cuts their throats. Pete provides a briefing on serial killers in general but Murphy snaps at him, saying to get on with the case. Pete explains that two months ago, Eric settled down and stopped trying to commit suicide. Now they figure that Eric will go after his last victim, Candy Farrell, who survived and testified against him. Walker assigns Murphy to watch the girl and Murphy balks, but Walker orders her to take on the assignment. Afterward, she says that she asked out of Homicide but Walker says he has no choice. Pete wants MacGyver to check Eric's cell and determine how he came up with a drug to simulate a heart attack. MacGyver, distracted by Murphy's distress, agrees.
Eric goes to an adult arcade to find Candy. She hasn't been working there for a year and doesn't have a contact number. The manager, Fontaine, agrees to give Eric a message if Candy shows up. Once Eric is gone, the arcade manager checks a newspaper article on Eric and then calls the police. Murphy and Sweeney investigate and discover Eric's supposed phone number is a 900 sex line. They ask Fontaine to help locate Candy.
MacGyver goes to the state hospital and the head orderly, Charles Davis, takes him to Eric's cell. Charles lets Zito talk to his lawyer on a cell phone, unaware that he's actually talking to Eric. Eric assures him that Zito's plan is working perfectly and the police are investigating the arcade. Zito confirms that Murphy is in charge of the investigation, then tells him where to find the phosphorous timer. However, he warns Eric that he has to be ready before midnight and kill Murphy in a very special way, or he'll lose his place in Valhalla. Eric assures him he'll be on time.
MacGyver goes over Eric's cell and discover sketches of Viking warriors. Zito talks to him from across the hall, saying that solving Eric's cunning riddle and warning that MacGyver will have to be intellectually up to the task. He calls MacGyver by name and then goes back to his reading. MacGyver finds more sketches and some powder contained within a plastic wrap. He tests the powder with water and recognizes the odor. Looking around the cell, MacGyver notices a hardcover book with a plastic cover that matches the plastic wrap. Outside, Charles explains that Zito murdered nine people and has degrees in genetics and psychiatry. MacGyver asks Charles to check with Maintenance to see if there were any times when Eric's toilet was clogged, and the library to get a complete list of books.
Back at the station, MacGyver talks to Murphy, who apologizes for snapping at him the previous day. When he asks if they have any leads on Eric himself, she irritably tells him they're not working that end of the case. MacGyver then goes over the books that Eric checked out from the library and finds one with a missing piece of cover. The checkout card reveals that Zito had it last. The maintenance logs show that Zito had a clogged toilet but Sweeney wonders why Zito and Eric would have anything to do with each other. He directs MacGyver to Murphy, noting she's the one who captured Zito. MacGyver shows the information to Murphy, who tells him to take it to Captain Walker because she's out of Homicide. He pushes her on the matter and she admits she got into Zito's head to track him down. Murphy warns that if she has any contact with Zito again, she'll lose that mind.
That night, Fontaine calls the station and asks for Murphy. He tells her that he's located Candy at a porno studio and gives her the address. Once he hangs up, Eric holds a knife to his throat and congratulates him.
MacGyver returns to check Eric's cell and finds Zito waiting for him. He's interested to hear MacGyver's theories and MacGyver obliges, explaining that Eric used the piece of plastic from the cover to make a crude cup, and then obtained industrial cleaner, diluted it through tissue paper, and created a non-fatal alkaline cocktail to simulate a heart attack. MacGyver demonstrates how Zito transferred it to Eric by pulling it over on a piece of toilet paper. He confronts Zito who admits he has nothing to lose.
Murphy and Sweeney go to the address Fontaine provided. The door is unlocked and they hear music upstairs. They find a metal door and Sweeney goes in first. Eric kills him with a Viking battle axe and then grabs Murphy, saying she's the chosen one. At the mental hospital, MacGyver wonders why Zito would have helped Eric escape. Zito refuses to help him, saying the stakes are very high and time is running out. He says that by midnight, in one hour, the riddle will solve itself.
Eric tapes a book of matches to the lower door to the warehouse and gently closes it. After turning on two tanks of propane gas, he goes upstairs to where he has Murphy tied up and gagged. He bolts the door and removes some phosphorous from a container. It ignites on contact with air and he smiles in satisfaction, then snuffs it out.
With 40 minutes remaining, MacGyver goes over Eric's files and Zito admits he "counseled" him. Eric was obsessed with Viking burial rites. Zito claims that he's bored and says the game is over.
Eric puts phosphorous and liquid in milk jugs and hangs them it from the ceiling. He then overdoses on sleeping pills and tells Murphy that it's almost time. He then punctures holes in the sides of the milk jugs, letting the containment fluid slowly leak out.
With 25 minutes left, MacGyver finds sketches of Viking maidens being burned and ascending to Valhalla. He realizes that Zito loaded up Eric on Viking symbology to encourage him to seek out Valhalla and cleanse his soul of guilt. To achieve Valhalla, Eric has to sacrifice a handmaiden, and Zito encouraged Eric to believe the doctor was a Valkyrie, who would choose Murphy. MacGyver realizes that Zito has set the whole thing up to get revenge on Murphy, the woman who put him away. Zito tells him that time's up and turns away. MacGyver goes out and calls the station. He tells Cooper and Wyatt to meet him at the warehouse and to bring the fire department.
Eric begins to doze off and lies down to await his fiery death.
MacGyver gets to the warehouse and smells the gas. As Cooper and Wyatt arrive, MacGyver goes through garbage cans and finds an empty oil can and a rag. He covers the rag with motor oil and uses it to see through the translucent door window. MacGyver spots the propane tanks and finds the matches. He uses a soda straw to soak the matches in water and opens the door. They shut the gas off and find Sweeney's corpse. Upstairs, the phosphorous ignites and MacGyver runs upstairs. He has Wyatt get his door buster and attaches it to the door while the firefighters arrive. This time the device works and MacGyver gets inside and unties Murphy, then gets her to safety. Wyatt gets the unconscious Eric out and they watch as the firefighters put out the blaze.
Eric is returned to his cell at the state hospital. Zito looks at him in disappointment and MacGyver tells the doctor that his plan failed. Murphy enters and confronts Zito, and her fear. She smiles and looks him in the eye.