A young stay-at-home mom writes a revolutionary mathematical proof and immediately attracts the attention of Samantha Carter at Stargate Command - not only because the proof could provide the solution to a tricky experiment that Dr. Rodney McKay has been conducting in Atlantis, but because this woman is Jeanie Miller, McKay's estranged sister.
McKay and Jeanie haven't spoken in four years, ever since she disappointed him by starting a family rather than pursuing a promising career in theoretical physics. Even now, despite Carter and McKay's invitation, Jeanie's not interested in helping out on a top-secret project - especially if her insensitive brother's involved. Finally, in frustration, McKay simply beams her up to the Daedalus in Earth orbit. Astonished, Jeanie finally agrees that this is worth taking a brief vacation from her beloved husband and daughter.
Jeanie and McKay travel to Atlantis, where McKay is designing a full-fledged zero-point-energy device. It generates much more power than the smaller-scale Zero Point Modules that Atlantis already possesses, but it also generates volatile exotic particles, which can destroy a star system - or worse - when they reach dangerous levels.
McKay and Jeanie apply her theories about bridges between parallel space-times to draw zero-point energy from a different universe, so that the exotic particles won't affect this universe at all. Though Jeanie worries that the particles might pose a threat to the other universe, she acknowledges that the odds of contacting another inhabited universe are astronomical.
Unfortunately, her fears come true. During the first test of the device, a second Dr. McKay, who calls himself Rod, materializes in Atlantis, having risked a daring journey between universes to warn the team that exotic particles from their test are about to tear his entire universe apart.
Rod is just as smart as McKay, but more confident and caring. Sheppard, Ronon and Teyla merely offend McKay by preferring Rod's company, but Jeanie breaks her brother's heart by doing the same - not that McKay knows how to admit that to her. Faced with a mirror image of his better self and a constant reminder of the sibling relationship he's poisoned, McKay must still cooperate with both Rod and Jeanie to solve the crisis. And they'd better hurry, because the Atlantis team in Rod's universe has thought of a way to save themselves - but only by reversing the problem entirely and creating a deadly rupture in this universe...