Henry and Grant have their first successful engine test with the bridge device, but when Grace enters, Grant leaves the married couple to do their own celebrating. Henry gets cold feet though, and leaves Grace perplexed. Carter has his own love woes, Allison is being wooed by Grant. But everything's rosy for Fargo, who has prepared an entire day for the visit of Claudia Donovan, an official from Warehouse 13, who's in town to pick up the neutralizing goo upon which her organization depends. Just as she's noting how normal Eureka seems, giant redwoods sprout up, fully grown, in the center of GD headquarters. Claudia stays to help unravel the mystery.
Across town Jo collapses ironically while having a debate with Grant (who's in the process of quitting smoking with nanorobots designed by Allison) about the merits of healthy food with Jo. They rush her to GD and discover a bullet intact in her chest. Allison saves her, but how the bullet got lodged in Jo's chest is a mystery. Carter suggests teleportation as an explanation for the bullet and the trees, but Jo, looking at the bullet, notes that the design was from before 1950.
As Carter pursues the teleportation lead at Cafe Diem, the air begins shimmering and Carter evacuates the cafe just as a 1940s era plane appears in the wall. Claudia, realizing that all the strange objects are from another era, deduces that they could be dealing with time travel, at which Henry sends her and Fargo to search for clues in the field while he debriefs Carter on the bridge device. While there, Allison and Grant start having terrible coughing fits. The two conjecture that if something were pulled through a wormhole, resultant exotic particles might create a massive magnet, pulling other items from 1947 into the present and destroying each time. They realize that all the appearing objects are from Eureka circa 1947, and set about seeking the magnet. As Grant's coughing gets worse and he collapses, they come to the terrible realization that the molecules of Grant's body are the magnet, and if they don't stop it, he will die and Eureka will be destroyed.
Claudia and Fargo, following a radiation signature, find themselves in a new-grown minefield. Claudia panics and steps right on a mine and if she steps off it, it will explode. Fargo works out a delay, but as they leap from one, they land on another. The only upside is that being stuck there indefinitely would give them a little make-out time. But Jo gets an update of the explosion, and Carter rushes to their rescue.
The team theorizes that Warehouse 13's "goo" could purge Grant of the exotic particles, neutralizing his magnetism. So they set up the goo on an IV, but it only makes the particles stronger. Inspired, Allison recalibrates her nanorobots to remove the exotic particles. The process nearly kills Grant, but he pulls through in the end, and the anachronistic items go back to their place ... or time.
After this brush with death, Henry decides he wants to spend a little time with Grace, Allison relents to having a drink with Grant, and Carter catches up with Jo, who is encouraged that Zane sat at her bedside during her emergency. Meanwhile, to be safe, Fargo and Grant decide that the bridge device should go with Claudia to Warehouse 13.
The cast and creators of "Eureka" help you reacquaint yourself with your favorite characters.