McKay is urgently needed back on Earth - Carter has a problem only he can solve. That problem turns out to be named Jeannie Miller, McKay's estranged younger sister. It turns out that she has solved the problem of extracting vacuum energy from a parallel universe in her free time with her daughter's finger paints - the same problem that led to McKay blowing up a solar system last season. The trouble is she's Happily Married and has absolutely no interest in signing a confidentiality agreement that will make her work the protected property of the US Military.
McKay manages convince her to come back to Atlantis with him, but not before she reveals that his first name is in fact Meredithnote . They call ahead to Zelenka to get everything at Atlantis prepped by the time they get there. (Nobody seems to raise the question of whether running the solar-system-destroying experiments from Atlantis is a good idea). Despite the constant sparring between the siblings, everything works perfectly, and they leave it running at low levels to see if anything goes wrong before ramping it up. Naturally, it doesn't take long before Zelenka is frantically radioing in to say that a foreign object has appeared in the midst of the containment field.
The foreign object turns out to be named Rod McKay - our Rodney's alternate universe counterpart! And gee, what a nice guy he is - he even has a leather jacket (he's just that cool)! It seems that the experiment, while successful on this side, ripped a hole in the fabric of spacetime which is threatening to swallow up alternate!Atlantis. Followed by the rest of the universe. Weir obligingly orders the experiment shut down, and Rod proceeds to charm everybody, to Rodney's growing consternation.
Suddenly, the experiment starts up again. It turns out just unplugging it didn't fix everything. The science team on the other side has a plan that will close the rift by destroying our universe. All three McKays (Rod, Rodney, and Jeannie) put their heads together and find a solution to save both universes. Unfortunately, this solution involves both sending Rod back (perhaps fatally) and severely depleting the ZPM. There's nothing else for it, though, and they send Rod on through. Just shutting down the bridge between the two universes uses up most of the ZPM's power...but McKay has to push it just a little farther to ensure that Rod survives to the other side, and the ZPM is wholly depleted. Weir decides not to fire McKay for leaving Atlantis completely defenseless, and Jeannie returns to Earth via the Daedalus.
- The Ace: Alternate universe "Rod" McKay
- Always Someone Better: McKay gets a double dose of this. First, his brilliant sister easily solves a physics problem that stumped him in her spare time, and later his own alternate-reality counterpart.
- Alternate Universe: Where Rod is from.
- Apocalypse How: Two universes are threatened with destruction by the experiment.
- Aw, Look! They Really Do Love Each Other: Sibling version. Rodney and Jennifer reconcile at the end and promise to keep in touch more often.
- Brilliant, but Lazy: Jeannie, at least from Rodney's warped perspective. Part of their estrangement is due to him being utterly unable to understand why she gave up a promising career in physics to get married and have kids.
- Call-Back: Rodney recorded the message to his sister back in "Letters from Pegasus".
- Carter brings up the time he blew up three fifths of a solar system.
- Department of Redundancy Department: The computer screen reads "ZPM module depleted" - which expands to "Zero Point Module module depleted"
- Embarrassing First Name: McKay's first name turns out to be Meredith — to everyone's amusement but his.
- And perhaps the even more Embarrassing Nickname "Mer"
- Expendable Alternate Universe: Subverted; the techies have no qualms doing great damage to an alternate reality until they find out that life also exists in it (a chance that was considered astronomically small).
- It's implied that Rod may have been working on something similar in his own reality and we later see in "The Daedalus Varations", that yet another Rodney McKay toyed with an alternate reality drive, suggesting that the events of this episode likely played themselves out in multiple realities.
- Family Versus Career: Jeannie is a straight example; this trope is the reason for her four-year estrangement from McKay. Jeannie left a promising career in physics to take care of her family, while McKay insists she made the wrong decision and belittles her choices. She repeatedly states throughout the episode that she stands by her decision (and the whole plot is kicked off by the fact that she's still smarter than McKay even though she's been out of "the game" for a while.) By the end of the episode, McKay has finally come to accept it and they reconcile.
- Foreshadowing: Rodney makes a reference to an "intergalactic gate network". That network eventually is shown a season later in Midway
- Girl of the Week: Sheppard may have had thoughts of treating Jeannie like this, but McKay shuts him down preemptively and points out that she's both his sister and married.
- Hot Scientist: Jeannie
- Hypocritical Humor: McKay warns Jeannie against Zelenka's ego.
- Meaningful Echo: Early in the episode, Jeannie calls Rodney on never communicating, even for holidays: "Just two or three times a year? Are you happy? Are you okay? That would have been enough!" At the end, Rodney admits he's wrong (a very rare occasion), and as the two tearfully embrace, Rodney asks, "Are you happy? Are you okay?"
- Million-to-One Chance: Carter and McKay realize the project could have disastrous consequences for whatever Alternate Universe the exotic particles get dumped into, but figure that the chances of randomly selecting a universe that's inhabited are "astronomically small". You'd think that with the number of Alt-SG1's they've already encountered, they'd know better.
- My Sister Is Off-Limits!: Rodney makes it very clear he does not want Sheppard hitting on his (married) sister. He gets riled when Sheppard simply introduces himself to her.
- McKay: She's married and she's my sister.
Sheppard: I was just saying hi.
McKay: I know what you were doing— I've seen that look before, Kirk.
- Nice Guy: Rod. Although the rest of the team admits he did get irritating after a while.
- Nice Job Breaking It, Hero!: This time, McKay nearly destroys the Universe - and not just this one.
- Not to mention completely depleting the ZPM - albeit semi-intentionally, in the name of getting Rod back to his universe.
- Other Me Annoys Me: Big time. And eventually annoys everyone else, too.
- Pet the Dog: After McKay has spent the entire episode being completely overshadowed by not only his younger sister but his far pleasanter alternate universe self, the rest of the team admit at the very end that they found "Rod" increasingly creepy and annoying as time went on, being far more happy with the McKay they've already got.
- Further, McKay realises what a jerk he's been to his sister and that she has every reason to prefer Rod, who's far nicer, to him, and starts treating her with more respect and accepts her life choices.
- Shout-Out: McKay once again compares Sheppard to Captain Kirk.
Rod: I guess there's nothing left to say but "there's no place like home."
- There are a few references to The Wonderful Wizard of Oz, including Rod's parting words.
- Tempting Fate: See Million-to-One Chance.
- You Are Better Than You Think You Are: Rod gives a speech to McKay along these lines, pointing out that he actually envies McKay for his Brutal Honesty.
- We Want Our Jerk Back: Consensus is that an outgoing McKay is actually worse than an aloof McKay.