The Department of Justice arrives to investigate Grace, while Henry struggles with his conscience and Eureka is cut off from the world by a malfunctioning experiment that threatens to eradicate the town’s populace.

While on patrol together, Ben and Hal try to help a young boy and wind up having their bikes stolen. The hunt to recover them leads them to the discovery of something much bigger. Also, Matt is put in danger, and Weaver is reunited with a family member.

Harvey bristles at Daniel’s involvement on a lawsuit he’s handling, but their battle to one up each other threatens the case, leaving Mike caught in the middle.

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

The 2nd Mass moves into an airport hanger. The Berserkers plan to get rid of Tom. While dealing with a loss of one of its own, the 2nd Mass receives a promise of hope from a mysterious visitor.

Kagame’s return begins with a reset of Liber8’s agenda, away from violence and towards blending into the community. But there may be a glitch – there is the possibility that if their ancestors living in this time are murdered, they might cease to exist. A test of this theory is devised with Kiera and her grandmother as the guinea pigs.

As Hardman makes it clear he’s back to stay, Jessica tasks Harvey with helping shore up support for her within the firm – but Harvey’s penchant for winning may create more enemies than friends.

While under the influence of a smart drug, Carter becomes a genius like everyone else in town and starts to change the laws of physics. Unfortunately, his condition soon endangers all of Eureka, including Allison’s brother who is in town just to meet his new brother-in-law.

Weaver is still in command of the 2nd Mass, while Anne continues to serve as medic. They have seen many battles and are a much leaner unit now. Pope has taken command of the Berserkers, a renegade group within the 2nd Mass that Weaver doesn’t fully trust.

Tom is haunted by memories of his time with the aliens. Pope plants seeds of doubt about him among the group. Ben’s hatred for skitters grows. The 2nd Mass locate a bridge that will help them cross the river to safety ahead of an impending attack.