The Phoenix Foundation has a contract to provide new technology for the police. Pete and MacGyver are participating in a stakeout of a crack house with a police to test their new door opening explosive device. They watch a prostitute make a buy and then the police launch their assault. MacGyver sets up his device on the rear door's gate but one of the four charges doesn't go off. One cop, Cooper, breaks down the door and MacGyver and another cop, Sweeney, try to stop the dealers from destroying the evidence. One dealer flushes the crack down the toilet. However, the cop in charge, Murphy, has put Wyatt underneath to intercept it. The remaining dealer tries to destroy the crack in the garbage disposal, but Murphy herself arrives and shoots out the disposal and recovers the crack.
As they return to the station, Murphy complains about the device's failure and expresses her preference for the old-fashioned tank. MacGyver warns her that the tank was what forced the city government to call in Phoenix to come up with an alternative, but Murphy refuses to give him a second chance.
At the state mental hospital in Alameda, one prisoner, Dr. Zito, talks to the prisoner across the hallway, Eric Cross. Zito assures Cross that "she" will come and be Eric's blessed messenger and take him to Valhalla. Eric lies down and then becomes ill. Zito calls for the nurses, who conclude that Eric is having a heart attack. They take him to the infirmary where he kills one nurse and escapes.
MacGyver is testing his door-busting device without success when Captain Walker calls MacGyver and Pete in to work on the task force to hunt down Cross. Murphy begins snapping at everyone as Walker goes over Cross' file. He stalked "loose women" and cut their throats until he was incarcerated. Two months ago, Cross settled down and stopped trying to commit suicide. They anticipate that Cross will go after his last victim, Candy Farrell, who survived his attack and testified against him. Walker orders Murphy and Sweeney from Narcotics to track her down. Murphy tries to refuse the assignment but Walker doesn't give her a choice. Meanwhile, Pete sends MacGyver to the hospital to figure out how Cross faked the heart attack.
At an adult arcade, Cross speaks with the manager and asks after Candy. The manager takes Cross' phone number and agrees to tell him if he comes up with anything. Once Cross leaves, the manager calls the police. Murphy and Sweeney investigate but discover Cross gave the manager a fake number.
At the hospital, the head orderly takes MacGyver to Cross' cell. Zito receives a phone call from his lawyer, but it's actually Cross, who is watching Murphy from across the street. Zito tells him to follow the plan and use the phosphorous timers before midnight. Once Cross kills Murphy, he'll gain a place in Valhalla.
MacGyver finds Cross' cell filled with sketches of Viking warriors and funeral rites. Zito talks to him from across the hallway, addresses him by name, and wonders if MacGyver's up to the challenge. MacGyver finds a piece of transparent plastic folded into a cup and filled with traces of white powder that reacts to water. MacGyver talks to the orderly, who explains that Zito has degrees in psychiatry and genetics, and killed nine people before he was captured. MacGyver asks him to check the maintenance records to see when Cross' toilet was clogged, and gets a list of Cross' books from the prison library.
Back at the station, MacGyver talks to Murphy. She apologizes for snapping at him but grows angry again when MacGyver asks about her Cross. He goes over Cross' books and finds one with a piece of missing plastic from the cover. According to the library logs, Zito last checked out the book. The maintenance log shows that Zito had a clogged toilet several days ago. Sweeney tells MacGyver to talk to Murphy about Zito. She refuses to talk about it at first but when MacGyver presses her, she explains that she's the one who brought Zito to justice, and that she had to get inside his head to do it. Now she worries that if she deals with Zito again, she'll go nuts.
The arcade manager calls Murphy and tells her he's located Candy at a porno studio in a warehouse. He gives her the address and hangs up… as Cross holds a knife to his hand.
MacGyver goes back to Cross' cell. Zito asks if he has a theory and MacGyver explains that Cross made a cup out of a piece of paper, and then drank diluted industrial cleaner which caused alkalosis, simulating a heart attack. Zito points out that Cross doesn't have the knowledge to distill industrial cleaner, and MacGyver explains that Zito created it and then used a roll of toilet paper to transfer it across the hallway to Cross.
Murphy and Sweeney go to the warehouse, going up a flight of stairs to a metal door. Cross is waiting and kills Sweeney with an axe, then takes Murphy prisoner.
MacGyver tries to figure out why Zito helped Cross, but the doctor merely says that time is running out and in one hour, at midnight, the riddle will solve itself.
Cross tapes a book of matches to the ground-floor entrance door of the warehouse and then turns on two tanks of propane gas. He goes upstairs to where he has tied Murphy to a pillar, then starts removing phosphorous from a bucket filled with water.
With 40 minutes left, MacGyver goes over Cross' sketches while Zito says that he counseled his fellow prisoner. MacGyver notes a common theme of funeral pyres in Cross' sketches but Zito says that he's bored and the game is over.
Cross hangs up several milk jugs filled with water and then puts phosphorous inside of them. He takes an overdose of sleeping pills and then punches holes in the milk jugs, letting the water slowly leak out.
MacGyver notices that many of the sketches show a Viking handmaiden being burned, and the Valkyrie sent to take them to Valhalla is made. He concludes that Zito convinced Cross he could purge his sins by achieving Valhalla. To do so, Cross had to sacrifice a handmaiden chosen by a Valkyrie, and Zito set himself up in the role. Zito engineered the whole thing so that Cross would kill Murphy and give the doctor his revenge. Zito warns MacGyver that his time is up and goes back to his cell bed. MacGyver calls Cooper and Wyatt and tells them to meet him at Murphy's last reported address.
Cross lies down and waits for his death and ascension.
MacGyver arrives at the warehouse and smells the propane gas. Using motor oil, he makes the translucent window transparent long enough to spot the propane tanks. Using a soda straw, he douses the matches in water and opens the door safely. The upstairs door is locked shut and MacGyver uses his door-busting device on it. As the phosphorous ignites, the device works successfully, and MacGyver gets Murphy out while Cooper and Wyatt rescue Cooper.
Later, Cross is returned to his cell, and MacGyver informs Zito that his plan failed. The doctor wonders how Murphy is taking it, and the detective comes in and looks him in the eye.