WARNING: This article contains spoilers for this week's episode of Star Trek: Discovery, "Choose Your Pain," which premiered Sunday.
Contrary to its title, "Choose Your Pain," this week's episode of Star Trek: Discovery seemed poised to end happily for everyone involved -- well, everyone not named Harry Mudd: Captain Lorca escaped from the Klingons, the tardigrade was set free, and Lt. Stamets and Dr. Culber, previously positioned as workplace rivals, were revealed to be a couple. But their shared moment of domesticity in the episode's final moments -- brushing their teeth together before heading off to bed -- was punctuated by a moment lifted straight from a horror film as Stamets' smiling reflection lingered after he'd walked away.
What should we make of that?
The most likely explanation -- oh, heck, perhaps the only explanation -- is that the reflection is the Stamets from the mirror universe, a parallel universe introduced in the beloved Star Trek: The Original Series Season 2 episode "Mirror, Mirror." In it, a transporter malfunction (surprise!) causes Kirk, McCoy, Scotty and Uhura to materialize on the Imperial Starship Enterprise, a darker version of the vessel they know, where officers assassinate their superiors in order to gain higher ranks, torture is used as a form of discipline, and (most sinister of all) Spock has facial hair. Their barbaric counterparts appear in their place aboard the USS Enterprise but, so we're told, that universe's Spock immediately recognized they were doppelgangers, and threw them in the brig.
Kirk & Co. avoided assassination and returned to their universe, with help from bearded Spock, and returned safe and sound to Enterprise, but the franchise was far from finished with the mirror universe, which was revisited in five episodes of Deep Space Nine and in two episodes of Enterprise, a TOS prequel.
Within the mirror universe, the United Federation of Planets is replaced by the brutal Terran Empire, which began when first contact with Vulcan was interpreted an alien invasion. The first Vulcan to step foot on Earth was shot (by Zefram Cochrane, no less), as recounted in the Enterprise episode "In a Mirror, Darkly," and the ship raided for its advanced technology, allowing the Terrans to expand well beyond the planet, conquering the races they encountered.
We've known for about a month that Discovery would return to the mirror universe, although given the drama's serialized nature -- far more so than any previous Star Trek -- it was difficult to imagine how. With "Choose Your Pain," we got our first indications.