SPOILERS! SPOILERS! SPOILERS!
I have noted before and often that I am not a fan of time travel. But if it has to be done, then this is the way I like it, and is the only way that makes sense to me: that when incidents in the past are changed, it alters the original timeline. The time travel stories I hate are the ones that involve paradoxes of any kind. I prefer Back to the Future II's temporal theory to that offered by Bill and Ted (and don't even get me started on the morass if inconsistency that is The Terminator chronology). None of this "I guess I DID steal my Dad's keys" or somehow being the cause of your own conception. I like pure, straightforward, someone steps on a butterfly and now dinosaurs still rule the Earth. It gives me way less of a headache.
Which is why I'm gonna roll with this Mirror Universe Eureka quite happily, 'cause for all that time travel annoys the hell out of me, at least they're doing it right. Of course, Eureka has done the alternate timeline thing before, when Henry, distraught over the loss of the love of his life, changed the course of human events in order to bring her back from the dead and then almost caused the fabric of the universe to unravel entirely.
So... ah, are we worried about that, here, at all? Apparently, not too much. So, okay.
So much is different in this alternate timeline (Fargo and Jo have new jobs, Carter and Tess are still together, Henry's married, Alison's son Kevin is no longer autistic), but, as Henry and Carter note: "The more things change..." "... the more they stay the same."
This week's disaster starts out with Tess's pet project going awry.(It's a huge mechanoid spider-like thing, and is pretty much the coolest looking experiment to ever go awry in this town of awry-going experimentation.) The problem is, of course, caused by Fargo, and fixed by Henry and Carter -- with a little assistance from the cause of all these parallel dimension shenanigans, 1947 runaway Dr. Trevor Grant.
We end with three certainties. The alternate timeline is here to stay, the six affected parties -- Carter, Alison, Jo, Fargo, Henry and Grant -- have to keep their adventure a secret (to avoid being "isolated and sanctioned" by the military), and Jack and Tess are moving in together. Hmmm. Look forward to seeing where you go with all this, Eureka! It really is quite ingenious. But really, no less than I'd expect from America's smartest little town.