Carter, arriving home from 1947, is surprised to find Tess at his house, happy and in love with him, when Kevin walks in, talking and independent. Jo is less lucky in love - Zane wants nothing to do with her. Henry, meanwhile, discovers he's mayor of Eureka.
In the middle of town, Carter and Allison are amazed to find Dr. Grant - who's very disappointed at the lack of progress since his day. His presence makes them realize what a broad impact their journey could have had. They talk to Henry and, despite Allison's protests at possibly losing Kevin again, make a plan to send Dr. Grant back in time and fix whatever rifts they may have caused. But Henry deduces that Grant manually retooled the bridge device to get himself to the future, making him a threat to their work. After some time in the jail cell, he confesses to the crime.
Tess's astronautic probe, meanwhile, is out of control, and no one knows why - but Tess blames Zane. Carter and his new deputy, the android Andy, find it in the woods. As Tess is repairing it, it explodes in a giant ball of light, sending Tess flying and badly injuring her. When Carter and Allison take her to be treated at Global Dynamics, they discover that Allison is the director of medicine, Fargo is head of GD, and Jo is head of GD security.
Henry is hard at work with the bridge device, and Carter tries to understand the nature of his relationship with Tess, but he's soon drawn away trying to discover who sabotaged Tess's probe. Later, as he talks to Henry about it, they reenter Henry's lab to discover the bridge device, damaged and inoperable. Unfortunately, the only expert they can bring onto the project is the deceptive Dr. Grant.
Grant soon discovers a cloud of antimatter positrons, the same force that destroyed Tess's probe, caused the damage. Bit by bit, they piece together the theory of positronic lightning, a project that alternate-reality Fargo created as a secret project for the U.S. Army. But they can't rest on their laurels for long, the lightning is out of control and must be neutralized. With Andy as a lightning rod, Carter will have time to safely ground the lightning and sap its power. They begin the plan, with Grant playing a key role and earning Carter's trust. The unlikely partnership succeeds, but Andy is badly damaged.
On a more personal level, Carter is conflicted – he learns that his alternate-reality self asked Tess to move in with him, and she said yes. The five time travelers gather for a post-mortem, and realize that telling anyone about the alternate time line could be extremely dangerous. With that, they make a pact to guard the secret with their lives.
Wormholes start popping up all over, threatening to rip Eureka apart unless Carter can stop them.