Returning from a mission in the Gamma Quadrant, a wormhole mishap sends Major Kira Nerys and Dr. Julian Bashir into the chaotic Mirror Universe at the beginning of the episode, where the Klingon-Cardassian Alliance governs supremely over captive Terrans on Terok Nor.
The best Mirror episode in Deep Space Nine, this season 2 gem masterfully expands the Mirror plot from The Original Series' "Mirror, Mirror" with unmatched tension, character subtlety, and world-building. Its gloomy plot and historical twists are commendable for being superior to later DS9 works like "Through the Looking Glass."
What truly sets “Crossover” apart is its full commitment to reimagining the entire DS9 ensemble within this darker sandbox, giving nearly every major character a Mirror reflection with a clear purpose and personality. Where later Mirror episodes sometimes feel like brief check-ins with a few fan-favorite alternates, “Crossover” presents the terrifying first glimpse of a fully realized society, one in which the Alliance’s cruelty has reshaped every familiar face.
1. Captain Kirk's mistake
About a century before during "Mirror, Mirror," Captain James T. Kirk had a big impact on Mirror Spock when his unintentional crossover led to modifications that made him the Terran Empire's commander-in-chief.
Kirk pushed Spock toward peace, reducing the once-barbaric Empire, just as the Klingon-Cardassian Alliance struck, capturing humanity and elevating Bajor to a significant Alliance member. This clever Prime Directive violation explores unintended consequences; Kirk's good intentions royally botched the Mirror timeline, setting up a slave-driven dystopia richer than any prior Trek Mirror tale.