Sudden muteness, human bioluminescence and faucets running blood cause many of Eureka’s citizens to believe that they are the victims of a biblical plague.

Global Dynamics sonic cleaning coincides with a phenomenon in which the citizens of Eureka develop a form of amnesia. Meanwhile, Allison goes into labor.

Beverly Barlow returns to town just as a parasitic bacterium threatens to destroy Eureka, by turning everything to gold and then into rust --- people included.

After hosting a movie party, Spencer is found the next day in the middle of a crop circle, claiming he was abducted by aliens; and then other people who attended the party start to go missing as well. Elsewhere, the legislator responsible for Eureka's funding visits the town, and gets dragged into a Sector 5 experiment...

Carter investigates an unusual car accident. Meanwhile, Stark, under the stress of completing a project on time, pressures two research teams to produce results.

It's space week in Eureka, and everyone is gearing up for the rocket race - except for Carter. General Mansfield is in town for the big Race to the Moon, and is debriefed on a new oxygen-generating fluid designed, called TAP, for colonizing Mars. When the question about someone using TAP to fuel their rocket, the whole town of Eureka is in danger and it is up to Kevin to save the day this time. Will he do it, or will Jack come back to see charred remains of his friends?

When Sara gives Andy a (as Zane calls it ) an EMO download to give him emotion, she inadvertently affects all Level 6 AIs. Including Tiny. She escapes while being transported to NASA and is on the loose. Meanwhile, Jack takes Kevin camping with Fargo and Grant and Jack is not happy that Grant is there. They argue on who is with Allison and who is not and they are interrupted by Tiny who has an attachment to a certain object of Kevin's.

The Eureka atmosphere becomes toxic when Department of Defense auditor comes to assess the team's relationships.

The citizens of Eureka attend the funeral of Susan Perkins, only to find out that she is in fact alive. Meanwhile the Sheriff moves into an underground bunker that is actually a 'smart house' controlled by a computer dubbed 'S.A.R.A.H., and begins to investigate a 'ghost' that has been appearing in town.

Taggart achieves a medical breakthrough while Fargo invents a device that bridges the divide between computers and human brains.

Dr. Whiticus creates an incredible "weather machine" intended to reverse the effects of global warming which instead stirs unexpected results.

The search is one for an escaped drone as Eva Thorne arrives to assist the clean up of damages which Global Dynamics has caused.

Carter and Allison investigate when a scientist mysteriously disappears from a sealed biosphere. Meanwhile, the "Fixer" tries to make Eureka's efforts more profitable.

Eureka's annual dog show is underway and is like none other. Meanwhile, Sheriff Carter experiences an earthquake; Zoe debates quitting the accelerated physics program; and Allison starts the process of fitting her wedding dress.

A search for a mummy ensues after a famed archaeologist opens the tomb of an Egyptian queen at Global Dynamics.

Love is in the air as Carter gets fed up with Grant's pursuit of Allison, Zoe conceals her love and Henry courts Grace. But all the emotions may have to be put on simmer when people in town start turning to stone.

When a smart fire experiment gets out of control, Carter and team discover that the Astraeus victims may be the cause.

Jack and Allison's perfect honeymoon is interrupted when sabotage is discovered in Eureka. Elsewhere, Holly faces another hurdle to remain intact.

A former resident returns to Eureka and creates mayhem which threatens Global Dynamics, prompting Stark to take action.

Falling space debris threatens Eureka residents and Carter turns to high school science fair participants to solve the problem. Specifically, he taps a student science project created to harness the sun's energy. Also, Zoe gets an eco-friendly automobile.

Carter's job is made more difficult when a super hero emerges. Meanwhile, Zoe and her friends are imperiled when Thorne makes a subterranean search beneath Eureka.

Carter finds himself chasing down creation of blob-ical like proportions when a series of escalating pranks turn deadly. Elsewhere, long-term rivals from Area 51 come to Eureka for a bowling tournament.

Henry's ship returns causing Carter to fear it maybe linked to the town's deadly electrical anomalies.

The first day on the job as temporary head of Global Dynamics proves daunting for Tess when a new Ice Age comes to Eureka; and a familiar face returns in the process.

When a prototype sets everyone into bad temper mode, the issue escalates and gets worse beyond the control of what Jack can handle. As Jack and Allison and Tess are the only sane people at Global Dynamics they have to devise a plan that will make everything calm and happy again. Meanwhile, Henry is helping Dr. Grant re-build the bridge that got them back to the new world except everything they touch disappears.