Fargo is dealing with his new reputation as a tyrant, as well as the pressure of answering to General Mansfield, who's demanding progress on a non-lethal weapons project. He and Jo head to the lab for a report, where they learn about a web-shooting device called an i-mine and a beam designed to pacify large crowds. Moments later, the i-mine traps Fargo against a wall. An investigation is launched to discover the source of the attack.
Tess and Carter bump into some relationship friction, but Carter doesn't have time for a talk - he has to solve the i-mine puzzle. The culprit is found to be Dr. Parrish, but why is murkier. Allison and the gang convince Fargo that he needs to revamp his image and change Global Dynamics culture by apologizing to the non-lethal weapons team. As he does, Parrish goes berserk, shooting a test gun around the lab and displaying a bizarre twitch in his eye. Jo and Fargo also become manic, and with great effort, Carter is able to control the crowd.
Grant, at a loss for what to do in this new world, convinces Henry to retool the bridge device. But before long, their experiment goes awry - everything they touch disappears.
Allison investigates the cause of the hostility, and discovers increased levels of adrenaline in Parrish. Before long, everyone except for Carter, Allison and Tess show symptoms, and they're escalating. Jo, growing enraged, pulls a gun on Carter, but he narrowly talks her down. The rest of the facility, however, is out of control and getting worse. Allison and Jo go looking for Tess, and discover Parrish, tranquilized and cuffed to a bed – but no Tess. When they see that Parrish is nearly non-human, they understand that everyone else is headed for the same fate. Jo, still in love, searches for Zane, and after a struggle manages to bring him back to the infirmary to safety.
Carter and Fargo find Tess, narrowly outrunning an angry mob, and it dawns on them that Carter, Tess and Allison weren't affected because they weren't in the GD building. Fargo deduces that the cause must've come from the non-lethal lab, and they narrow it down to the RSS ray. Tess thinks she can retune the beam to its original purpose of pacifying crowds. As Allison and Jo fend off an ever-stronger mob, Carter uses Fargo as bait to draw all the mad scientists into one location, where Tess unleashes the RSS beam in the nick of time.
Henry and Grant, oblivious to the GD crisis, frenziedly try to solve their own problem, until Grace walks in and says that she sprayed a dematerializing solution on Henry's old tools - a prank to get back at her husband for magnetizing her pots and pans.
With the physical threat at GD neutralized, it's time to make amends, and Carter has a big job ahead of him - he must tell Tess that he can't be with her. He encourages her to take the job in Australia, and she tearfully leaves his house.
The cast and creators of "Eureka" help you reacquaint yourself with your favorite characters.