A TOS reference in Star Trek: Discovery’s season 5 premiere suggests the USS Defiant may return from the Mirror Universe, confirming a fan theory.

WARNING: This article contains SPOILERS for Star Trek: Discovery season 5, episode 1, “Red Directive”

One line from Star Trek: Discovery season 5, episode 1, “Red Directive” makes a return to the Mirror Universe more likely, confirming a fan theory. Discovery last paid a visit to the Mirror Universe in the season 3 two-parter “Terra Firma”, in which the Guardian of Forever sent Philippa Georgiou (Michelle Yeoh) back to her last day as Terran Emperor. The return to the Mirror Universe was part of an elaborate test by the Guardian, now assuming the form of Carl (Paul Guilfoyle), to prove the positive impact of Georgiou’s experiences in the prime Star Trek timeline.

Now it seems that Star Trek: Discovery could be making a final return to the Mirror Universe in season 5. The final trailer for Discovery season 5 appeared to show L’ak (Elias Toufexis) standing in front of what appeared to be the symbol of the Terran Empire. Star Trek: Deep Space Nine revealed that the Terran Empire had fallen in the 24th century, but it’s certainly possible that it could have risen again in the 800 years between DS9 and Discovery. How the USS Discovery’s treasure hunt would lead to a return to the Mirror Universe is unclear, but there’s a small hint in season 5’s premiere.

Discovery’s Season 5 Premiere Made A Mirror Universe Theory More Likely

The Star Trek: Discovery season 5 trailer teased a tense gunfight aboard what appeared to be a 23rd century Constitution-class starship. This has led fans to speculate that it could be the USS Defiant from Star Trek: The Original Series, the starship that phased in and out of reality in “The Tholian Web”. That theory gets a further boost in Discovery season 5, episode 1, as Captain Saru (Doug Jones) outlines possible threats to the Federation in the 32nd century.

Saru reveals that the Breen Imperium and the Tholian Republic are potential threats to the Federation in Star Trek: Discovery season 5. It’s even possible that either of these classic Star Trek aliens could seek the Progenitors’ technology to gain an advantage over the Federation. Saru’s mention of the Tholians also gives Discovery a pathway back to the Mirror Universe later in season 5, perhaps affording the chance to settle some unfinished business.

Who Are Star Trek: TOS’ Tholians And How Do They Connect To The Mirror Universe?

In Star Trek: The Original Series, season 3, episode 9, “The Tholian Web”, the USS Enterprise encounters the stricken USS Defiant. The entire crew killed each other, driven insane by the effects of a spatial interphase caused by the non-humanoid Tholians. Star Trek: Enterprise revealed that it was the Tholians from the Mirror Universe that had actually dragged the USS Defiant out of their own time and reality, dragging it backwards into the 22nd century. Captain Jonathan Archer (Scott Bakula) of the ISS Enterprise NX-01 stole the Defiant, but the knowledge contained inside it led to a revolt aboard the ship.

The events of Star Trek: Enterprise‘s Mirror Universe episodes later informed Star Trek: Discovery season 1’s Mirror Universe arc. When the USS Discovery was dragged into the Mirror Universe by the machinations of Captain Gabriel Lorca, they used the information about the USS Defiant to return to their own reality. At the end of “The Tholian Web” – which takes place after Star Trek: Discovery – the USS Defiant is phased out of reality to parts unknown. Given that the Defiant had been dragged through both time and reality, it could have ended up in the 32nd century, providing Burnham with a bridge to the Mirror Universe.