|Star Trek: Deep Space Nine episode|
|Episode no.||Season 2|
|Directed by||David Livingston|
|Story by||Peter Allan Fields|
|Featured music||Dennis McCarthy|
|Original air date||May 16, 1994|
"Crossover" is the 43rd episode of the science fiction television series Star Trek: Deep Space Nine, and the 23rd episode of the second season. This episode was written by Peter Allan Fields and Michael Pillar, and directed by David Livingston. It aired on broadcast television on May 16, 1994.
Set in the 24th century, the series follows the adventures on Deep Space Nine, a space station located adjacent to a stable wormhole between the Alpha and Gamma quadrants of the Milky Way Galaxy, near the planet Bajor. In this episode, first officer Kira and Doctor Bashir have an adventure in Star Trek's Mirror Universe, a parallel universe setting introduced in the episode "Mirror, Mirror" of the original Star Trek series. It is the first of five Deep Space Nine episodes to involve the Mirror Universe.
After experiencing operational difficulties while traveling through the wormhole, Major Kira and Dr. Bashir find themselves in an alternate universe populated by exact doubles of the people in their own universe. In this universe, the wormhole is unknown, and Terrans (i.e., humans) are slaves; Deep Space Nine is a mining station run by the Intendant, Kira's counterpart, with Garak as her second in command and Odo overseeing the Terran slaves. As Bashir is sent to work in ore-processing, the Intendant tells Kira about James Kirk's visit to this universe a century ago; that incident led to the formation of a powerful alliance between the Klingons, Cardassians, and Bajorans, and the eventual conquest of the Terrans.
Bashir tries to talk Miles O'Brien, a fellow slave, into configuring the transporters to help Kira and Bashir return home, but the beaten, put-upon O'Brien is unwilling to risk the wrath of his superiors. Meanwhile, Kira almost succeeds in securing help from the bartender Quark, but he is arrested and executed by Garak for helping Terrans escape the station. Kira meets Benjamin Sisko's Mirror Universe counterpart, who receives better treatment than the other Terrans for carrying out piracy on behalf of the Klingon-Cardassian alliance.
The Intendant assures Kira that she has nothing to fear, and suggests they should become closer. Later, Garak tells Kira that he intends to kill the Intendant, and that he will let Kira and Bashir escape if she impersonates the Intendant and yields power to Garak; Bashir will be killed if she does not comply.
Kira tells Bashir they must find a way back to their runabout and make their escape through the wormhole. She tries to recruit Sisko's aid, but he is unmoved. That night, Garak prepares to put his plan into effect at a lavish party thrown by the Intendant. Meanwhile, an accident at the ore-processing plant gives Bashir the opportunity to escape, killing Odo in the process.
Bashir convinces O'Brien to help, but the two are captured and brought to face the Intendant, who sentences them to death. O'Brien makes an impassioned speech, telling the assembled crowd what Bashir has revealed about a universe where Terrans have respect and dignity. His words move Sisko, who turns on the Intendant and helps Kira and Bashir to escape. The two return to their universe, leaving Sisko and O'Brien's counterparts to fight for the rights of Terrans in their own world.
This was the first of five mirror universe episodes of Star Trek: Deep Space Nine: "Crossover", "Through the Looking Glass", "Shattered Mirror", "Resurrection", and "The Emperor's New Cloak".
In 2016, The Hollywood Reporter rated "Crossover" the 78th best episode of all Star Trek episodes. They note that this episode references the events in the Mirror Universe that occurred in the episode "Mirror, Mirror" and that this enhances the episode.
In 2020, the U.K. science fiction magazine and website SciFiNow ranked this one of the top ten episodes of Star Trek: Deep Space Nine, noting that actress Nana Visitor seemed to have fun playing her evil twin from the mirror universe. 
- "Star Trek: Deep Space Nine: "Crossover"/"The Collaborator"". AV Club. June 21, 2012.
- "Star Trek: A Guide to All the Mirror Universe Episodes". Den of Geek. Retrieved June 9, 2019.
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- Hollywood Reporter 'Star Trek': 100 Greatest Episodes
- Granshaw, Lisa (January 30, 2017). "Ranking every Mirror Universe episode of Star Trek". SYFY WIRE. Retrieved June 9, 2019.
- "Star Trek: Deep Space Nine's Must-Watch Episodes". Gizmodo. Retrieved June 2, 2021.
- "Deep Space Nine: Top Ten Episodes". SciFiNow. December 31, 2020. Retrieved June 8, 2021.
- "Star Trek: Deep Space Nine Vol. 22 - Crossover/The Collaborator". www.videocollector.co.uk. Retrieved February 22, 2021.
- "DVD Talk". www.dvdtalk.com. Retrieved February 22, 2021.
- Staff, TrekNews net (February 10, 2017). "[REVIEW] Deep Space Nine Complete Series DVD Box Set". TREKNEWS.NET | Your daily dose of Star Trek news and opinion. Retrieved February 19, 2021.
- Star Trek: Deep Space Nine - Season Two - IGN, retrieved February 22, 2021
- Star Trek: Deep Space Nine DVD set, volume 2, disc 6, selection 3.
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