With Allison going on maternity leave, Tess is now in charge of Global Dynamics. And Tess' first project as head of the company is to take possession of a special ice core which has been shipped all the way from Russia. Accompanying the ice are two familiar faces, Taggart and Zane. Jo isn't too pleased to see Taggart around again, judging from their past relationship, but when she tries to talk to Zane, he's strangely cold to her. Meanwhile, at Cafe Diem, Zoe is awaiting the results of her Student Career Assessment Test, and she's already looking at colleges after planning on starting a career in medicine. But her results say that she should look into a career in robotics. Zoe is disappointed, but Lucas gives her a robotics kit to test her skills.
Since the ice core was found in Russian territory, a Russian Commissar must check on the town's safety protocols and to make sure that Eureka is ready for any kind of emergency. Jack discovers that Eureka has an emergency pneumatic vacuum tube system which connects all the buildings in the town, as a way to physically send messages if all other forms of communication fail. When Jack goes into the room where the ice core is stored, he finds that there is much more ice than before, and that Taggart himself has been frozen.
Once Taggart is thawed out, he explains that the ice attacked him. And suddenly the ice is appearing to spread all over the Global Dynamics building. When Jack mentions that the ice has black streaks in it, Taggart says that those black streaks are from malformed ice crystals from the ice spreading out very rapidly. With everyone else working to stop the ice, Jack figures out that Zane isn't really helping out, so he decides to investigate. Jo points out that Zane wasn't acting like himself, and when the cup of coffee that Zane blows on freezes solid, it's clear that something weird has taken over Zane's body. After Zane collapses, he is taken for examination, where it's discovered that ice crystals are inside his body, which was causing the change in personality.
Taggart finds that there are fungi hiding in the ice core, and Jack figures out why Zane and Taggart were attacked differently. Zane was wearing a special type of cold-weather gear made out of a substance called Petro-Tek, while Taggart wasn't. Petro-Tek apparently is like a fertilizer for the fungi in the ice, and since all of the buildings in Eureka contain it, the fungi are spreading the ice all over the town. They can get rid of the fungi with a special fungicide, but they're having trouble figuring out how to distribute it throughout the buildings. But then Jack remembers about the tube system connecting the buildings, so an aerosol form of the fungicide is spread through the town via the tubes.
Taggart and Jack arrive at Cafe Diem in order to check on Zoe and the others, but they find no one in the place until they check the walk-in fridge. Since the fridge is at a regulated temperature, Zoe sent everyone inside. The fungicide manages to kill the fungi and melt all the ice away. Afterwards, Zoe manages to finish the robot, but Lucas says that she should trust her instincts instead. If her instincts told her that going into the fridge would be safer, then she should know that becoming a doctor would be better for her than going into robotics. Tess and Jack end up spending a quiet evening together in front of the fireplace.