“What Goes Around, Comes Around” is episode eighteen of season three of Eureka and the season finale.
Henry and Zoe’s boyfriend, Lucas, are examining the data from Henry’s probe, when they discover confirmation of the existence of Nemesis, a neutron star orbiting the sun (Nemesis is a theoretical companion to our Sun, but it is actually believed to be either a brown or red dwarf if it exists).
Fargo is in Cafe Diem saying how Nemesis means the Earth is doomed. Carter gets worried by this, and asks how much time they have. When he hears that it’s 2,000 years, he’s rather less concerned.
Back at Global Dynamics, an experiment into dark energy goes wrong (imagine; an experiment goes wrong, what a turn-up for the books!). Then, magnetic anomalies start popping up over Eureka, wreaking havoc.
Carter is also a bit down because Zoe is heading off to college and Tess has got a job offer – in Australia.